dchain/node/api_feed.go

package node

// Feed HTTP endpoints (v2.0.0).
//
// Mount points:
//
//	POST /feed/publish            — store a post body (authenticated)
//	GET  /feed/post/{id}          — fetch a post body
//	GET  /feed/post/{id}/stats    — {views, likes, liked_by_me?} aggregate
//	POST /feed/post/{id}/view     — increment off-chain view counter
//	GET  /feed/author/{pub}       — ?limit=N, posts by an author
//	GET  /feed/timeline           — ?follower=<pub>&limit=N, merged feed of follows
//	GET  /feed/trending           — ?window=h&limit=N, top by likes + views
//	GET  /feed/foryou             — ?pub=<pub>&limit=N, recommendations
//	GET  /feed/hashtag/{tag}      — posts matching a hashtag
//
// Publish flow:
//   1. Client POSTs {content, attachment, post_id, author, sig, ts}.
//   2. Node verifies sig (Ed25519 over canonical bytes), hashes body,
//      stores in FeedMailbox, returns hosting_relay + content_hash + size.
//   3. Client then submits on-chain CREATE_POST tx with that metadata.
//      Node charges the fee (base + size×byte_fee) and credits the relay.
//   4. Subsequent GET /feed/post/{id} serves the stored body to anyone.
//
// Why the split? On-chain metadata gives us provable authorship + the
// pay-for-storage incentive; off-chain body storage keeps the block
// history small. If the hosting relay dies, the on-chain record stays
// (with a "body unavailable" fallback on the reader side) — authors can
// re-publish to another relay.

import (
	"context"
	"crypto/sha256"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"log"
	"net/http"
	"sort"
	"strings"
	"time"

	"go-blockchain/blockchain"
	"go-blockchain/identity"
	"go-blockchain/media"
	"go-blockchain/relay"
)

// FeedConfig wires feed HTTP endpoints to the relay mailbox and the
// chain for read-after-write queries.
type FeedConfig struct {
	Mailbox *relay.FeedMailbox

	// HostingRelayPub is this node's Ed25519 pubkey — returned from
	// /feed/publish so the client knows who to put in CREATE_POST tx.
	HostingRelayPub string

	// Scrubber strips metadata from image/video/audio attachments before
	// they are stored. MUST be non-nil; a zero Scrubber (NewScrubber with
	// empty sidecar URL) still handles images in-process — only video/audio
	// require sidecar config.
	Scrubber *media.Scrubber

	// AllowUnscrubbedVideo controls server behaviour when a video upload
	// arrives and no sidecar is configured. false (default) → reject; true
	// → store as-is with a warning log. Set via --allow-unscrubbed-video
	// flag on the node. Leave false in production.
	AllowUnscrubbedVideo bool

	// Chain lookups (nil-safe; endpoints degrade gracefully).
	GetPost         func(postID string) (*blockchain.PostRecord, error)
	LikeCount       func(postID string) (uint64, error)
	HasLiked        func(postID, likerPub string) (bool, error)
	PostsByAuthor   func(authorPub string, limit int) ([]*blockchain.PostRecord, error)
	Following       func(followerPub string) ([]string, error)
}

// RegisterFeedRoutes wires feed endpoints onto mux. Writes are rate-limited
// via withSubmitTxGuards; reads via withReadLimit (same limiters as /relay).
func RegisterFeedRoutes(mux *http.ServeMux, cfg FeedConfig) {
	if cfg.Mailbox == nil {
		return
	}
	mux.HandleFunc("/feed/publish", withSubmitTxGuards(feedPublish(cfg)))
	mux.HandleFunc("/feed/post/", withReadLimit(feedPostRouter(cfg)))
	mux.HandleFunc("/feed/author/", withReadLimit(feedAuthor(cfg)))
	mux.HandleFunc("/feed/timeline", withReadLimit(feedTimeline(cfg)))
	mux.HandleFunc("/feed/trending", withReadLimit(feedTrending(cfg)))
	mux.HandleFunc("/feed/foryou", withReadLimit(feedForYou(cfg)))
	mux.HandleFunc("/feed/hashtag/", withReadLimit(feedHashtag(cfg)))
}

// ── POST /feed/publish ────────────────────────────────────────────────────

// feedPublishRequest — what the client sends. Signature is Ed25519 over
// canonical bytes: "publish:<post_id>:<content_sha256_hex>:<ts>".
// ts must be within ±5 minutes of server clock.
type feedPublishRequest struct {
	PostID         string `json:"post_id"`
	Author         string `json:"author"`               // hex Ed25519
	Content        string `json:"content"`
	ContentType    string `json:"content_type,omitempty"`
	AttachmentB64  string `json:"attachment_b64,omitempty"`
	AttachmentMIME string `json:"attachment_mime,omitempty"`
	ReplyTo        string `json:"reply_to,omitempty"`
	QuoteOf        string `json:"quote_of,omitempty"`
	Sig            string `json:"sig"` // base64 Ed25519 sig
	Ts             int64  `json:"ts"`
}

type feedPublishResponse struct {
	PostID         string   `json:"post_id"`
	HostingRelay   string   `json:"hosting_relay"`
	ContentHash    string   `json:"content_hash"` // hex sha256
	Size           uint64   `json:"size"`
	Hashtags       []string `json:"hashtags"`
	EstimatedFeeUT uint64   `json:"estimated_fee_ut"` // base + size*byte_fee
}

func feedPublish(cfg FeedConfig) http.HandlerFunc {
	const publishSkewSecs = 300

	return func(w http.ResponseWriter, r *http.Request) {
		if r.Method != http.MethodPost {
			jsonErr(w, fmt.Errorf("method not allowed"), 405)
			return
		}
		var req feedPublishRequest
		if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
			jsonErr(w, fmt.Errorf("invalid JSON: %w", err), 400)
			return
		}
		if req.PostID == "" || req.Author == "" || req.Sig == "" || req.Ts == 0 {
			jsonErr(w, fmt.Errorf("post_id, author, sig, ts are required"), 400)
			return
		}
		if req.Content == "" && req.AttachmentB64 == "" {
			jsonErr(w, fmt.Errorf("post must have content or attachment"), 400)
			return
		}
		now := time.Now().Unix()
		if req.Ts < now-publishSkewSecs || req.Ts > now+publishSkewSecs {
			jsonErr(w, fmt.Errorf("ts out of range (±%ds)", publishSkewSecs), 400)
			return
		}
		if req.ReplyTo != "" && req.QuoteOf != "" {
			jsonErr(w, fmt.Errorf("reply_to and quote_of are mutually exclusive"), 400)
			return
		}

		// Decode attachment.
		var attachment []byte
		var attachmentMIME string
		if req.AttachmentB64 != "" {
			b, err := base64.StdEncoding.DecodeString(req.AttachmentB64)
			if err != nil {
				if b, err = base64.RawURLEncoding.DecodeString(req.AttachmentB64); err != nil {
					jsonErr(w, fmt.Errorf("attachment_b64: invalid base64"), 400)
					return
				}
			}
			attachment = b
			attachmentMIME = req.AttachmentMIME

			// MANDATORY server-side scrub: strip ALL metadata (EXIF/GPS/
			// camera/author/ICC/etc.) and re-compress. Client is expected
			// to have done a first pass, but we never trust it — a photo
			// from a phone carries GPS coordinates by default and the client
			// might forget or a hostile client might skip the scrub entirely.
			//
			// Images are handled in-process (stdlib re-encode to JPEG kills
			// all metadata by construction). Videos/audio are forwarded to
			// the media sidecar; if none is configured and the operator
			// hasn't opted in to AllowUnscrubbedVideo, we reject.
			if cfg.Scrubber == nil {
				jsonErr(w, fmt.Errorf("media scrubber not configured on this node"), 503)
				return
			}
			ctx, cancel := context.WithTimeout(r.Context(), 60*time.Second)
			cleaned, newMIME, err := cfg.Scrubber.Scrub(ctx, attachment, attachmentMIME)
			cancel()
			if err != nil {
				// Graceful video fallback only when explicitly allowed.
				if err == media.ErrSidecarUnavailable && cfg.AllowUnscrubbedVideo {
					// Keep bytes as-is (operator accepted the risk), just log.
					log.Printf("[feed] WARNING: storing unscrubbed video — no sidecar configured (author=%s)", req.Author)
				} else {
					status := 400
					if err == media.ErrSidecarUnavailable {
						status = 503
					}
					jsonErr(w, fmt.Errorf("scrub attachment: %w", err), status)
					return
				}
			} else {
				attachment = cleaned
				attachmentMIME = newMIME
			}
		}

		// Content hash is computed over the scrubbed bytes — that's what
		// the on-chain tx will reference, and what readers fetch. Binds
		// the body to the metadata so a misbehaving relay can't substitute
		// a different body under the same PostID.
		h := sha256.New()
		h.Write([]byte(req.Content))
		h.Write(attachment)
		contentHash := h.Sum(nil)
		contentHashHex := hex.EncodeToString(contentHash)

		// Verify the author's signature over the canonical publish bytes.
		msg := []byte(fmt.Sprintf("publish:%s:%s:%d", req.PostID, contentHashHex, req.Ts))
		sigBytes, err := base64.StdEncoding.DecodeString(req.Sig)
		if err != nil {
			if sigBytes, err = base64.RawURLEncoding.DecodeString(req.Sig); err != nil {
				jsonErr(w, fmt.Errorf("sig: invalid base64"), 400)
				return
			}
		}
		if _, err := hex.DecodeString(req.Author); err != nil {
			jsonErr(w, fmt.Errorf("author: invalid hex"), 400)
			return
		}
		ok, err := identity.Verify(req.Author, msg, sigBytes)
		if err != nil || !ok {
			jsonErr(w, fmt.Errorf("signature invalid"), 403)
			return
		}

		post := &relay.FeedPost{
			PostID:         req.PostID,
			Author:         req.Author,
			Content:        req.Content,
			ContentType:    req.ContentType,
			Attachment:     attachment,
			AttachmentMIME: attachmentMIME,
			ReplyTo:        req.ReplyTo,
			QuoteOf:        req.QuoteOf,
		}
		hashtags, err := cfg.Mailbox.Store(post, req.Ts)
		if err != nil {
			if err == relay.ErrPostTooLarge {
				jsonErr(w, err, 413)
				return
			}
			jsonErr(w, err, 500)
			return
		}

		// Report what the client should put into CREATE_POST.
		size := uint64(len(req.Content)) + uint64(len(attachment)) + 128
		fee := blockchain.BasePostFee + size*blockchain.PostByteFee
		jsonOK(w, feedPublishResponse{
			PostID:         req.PostID,
			HostingRelay:   cfg.HostingRelayPub,
			ContentHash:    contentHashHex,
			Size:           size,
			Hashtags:       hashtags,
			EstimatedFeeUT: fee,
		})
	}
}

// ── GET /feed/post/{id}  [+ /stats subroute, POST /view] ─────────────────

// feedPostRouter dispatches /feed/post/{id}, /feed/post/{id}/stats,
// /feed/post/{id}/view to the right handler.
func feedPostRouter(cfg FeedConfig) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		rest := strings.TrimPrefix(r.URL.Path, "/feed/post/")
		rest = strings.Trim(rest, "/")
		if rest == "" {
			jsonErr(w, fmt.Errorf("post id required"), 400)
			return
		}
		parts := strings.Split(rest, "/")
		postID := parts[0]
		if len(parts) == 1 {
			feedGetPost(cfg)(w, r, postID)
			return
		}
		switch parts[1] {
		case "stats":
			feedPostStats(cfg)(w, r, postID)
		case "view":
			feedPostView(cfg)(w, r, postID)
		default:
			jsonErr(w, fmt.Errorf("unknown sub-route %q", parts[1]), 404)
		}
	}
}

type postHandler func(w http.ResponseWriter, r *http.Request, postID string)

func feedGetPost(cfg FeedConfig) postHandler {
	return func(w http.ResponseWriter, r *http.Request, postID string) {
		if r.Method != http.MethodGet {
			jsonErr(w, fmt.Errorf("method not allowed"), 405)
			return
		}
		post, err := cfg.Mailbox.Get(postID)
		if err != nil {
			jsonErr(w, err, 500)
			return
		}
		if post == nil {
			jsonErr(w, fmt.Errorf("post %s not found", postID), 404)
			return
		}
		// Respect on-chain soft-delete.
		if cfg.GetPost != nil {
			if rec, _ := cfg.GetPost(postID); rec != nil && rec.Deleted {
				jsonErr(w, fmt.Errorf("post %s deleted", postID), 410)
				return
			}
		}
		jsonOK(w, post)
	}
}

type postStatsResponse struct {
	PostID    string `json:"post_id"`
	Views     uint64 `json:"views"`
	Likes     uint64 `json:"likes"`
	LikedByMe *bool  `json:"liked_by_me,omitempty"` // set only when ?me=<pub> given
}

func feedPostStats(cfg FeedConfig) postHandler {
	return func(w http.ResponseWriter, r *http.Request, postID string) {
		if r.Method != http.MethodGet {
			jsonErr(w, fmt.Errorf("method not allowed"), 405)
			return
		}
		views, _ := cfg.Mailbox.ViewCount(postID)
		var likes uint64
		if cfg.LikeCount != nil {
			likes, _ = cfg.LikeCount(postID)
		}
		resp := postStatsResponse{
			PostID: postID,
			Views:  views,
			Likes:  likes,
		}
		if me := r.URL.Query().Get("me"); me != "" && cfg.HasLiked != nil {
			if liked, err := cfg.HasLiked(postID, me); err == nil {
				resp.LikedByMe = &liked
			}
		}
		jsonOK(w, resp)
	}
}

func feedPostView(cfg FeedConfig) postHandler {
	return func(w http.ResponseWriter, r *http.Request, postID string) {
		if r.Method != http.MethodPost {
			jsonErr(w, fmt.Errorf("method not allowed"), 405)
			return
		}
		next, err := cfg.Mailbox.IncrementView(postID)
		if err != nil {
			jsonErr(w, err, 500)
			return
		}
		jsonOK(w, map[string]any{
			"post_id": postID,
			"views":   next,
		})
	}
}

// ── GET /feed/author/{pub} ────────────────────────────────────────────────

func feedAuthor(cfg FeedConfig) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		if r.Method != http.MethodGet {
			jsonErr(w, fmt.Errorf("method not allowed"), 405)
			return
		}
		pub := strings.TrimPrefix(r.URL.Path, "/feed/author/")
		pub = strings.Trim(pub, "/")
		if pub == "" {
			jsonErr(w, fmt.Errorf("author pub required"), 400)
			return
		}
		limit := queryInt(r, "limit", 50)

		// Prefer chain-authoritative list (includes soft-deleted flag) so
		// clients can't be fooled by a stale relay that has an already-
		// deleted post. If chain isn't wired, fall back to relay index.
		if cfg.PostsByAuthor != nil {
			records, err := cfg.PostsByAuthor(pub, limit)
			if err != nil {
				jsonErr(w, err, 500)
				return
			}
			out := make([]feedAuthorItem, 0, len(records))
			for _, rec := range records {
				if rec == nil || rec.Deleted {
					continue
				}
				out = append(out, buildAuthorItem(cfg, rec))
			}
			jsonOK(w, map[string]any{"author": pub, "count": len(out), "posts": out})
			return
		}
		ids, err := cfg.Mailbox.PostsByAuthor(pub, limit)
		if err != nil {
			jsonErr(w, err, 500)
			return
		}
		out := expandByID(cfg, ids)
		jsonOK(w, map[string]any{"author": pub, "count": len(out), "posts": out})
	}
}

// feedAuthorItem is a chain record enriched with the body and live stats.
type feedAuthorItem struct {
	PostID       string   `json:"post_id"`
	Author       string   `json:"author"`
	Content      string   `json:"content,omitempty"`
	ContentType  string   `json:"content_type,omitempty"`
	Hashtags     []string `json:"hashtags,omitempty"`
	ReplyTo      string   `json:"reply_to,omitempty"`
	QuoteOf      string   `json:"quote_of,omitempty"`
	CreatedAt    int64    `json:"created_at"`
	Size         uint64   `json:"size"`
	HostingRelay string   `json:"hosting_relay"`
	Views        uint64   `json:"views"`
	Likes        uint64   `json:"likes"`
	HasAttachment bool    `json:"has_attachment"`
}

func buildAuthorItem(cfg FeedConfig, rec *blockchain.PostRecord) feedAuthorItem {
	item := feedAuthorItem{
		PostID:       rec.PostID,
		Author:       rec.Author,
		ReplyTo:      rec.ReplyTo,
		QuoteOf:      rec.QuoteOf,
		CreatedAt:    rec.CreatedAt,
		Size:         rec.Size,
		HostingRelay: rec.HostingRelay,
	}
	if body, _ := cfg.Mailbox.Get(rec.PostID); body != nil {
		item.Content = body.Content
		item.ContentType = body.ContentType
		item.Hashtags = body.Hashtags
		item.HasAttachment = len(body.Attachment) > 0
	}
	if cfg.LikeCount != nil {
		item.Likes, _ = cfg.LikeCount(rec.PostID)
	}
	item.Views, _ = cfg.Mailbox.ViewCount(rec.PostID)
	return item
}

// expandByID fetches bodies+stats for a list of post IDs (no chain record).
func expandByID(cfg FeedConfig, ids []string) []feedAuthorItem {
	out := make([]feedAuthorItem, 0, len(ids))
	for _, id := range ids {
		body, _ := cfg.Mailbox.Get(id)
		if body == nil {
			continue
		}
		item := feedAuthorItem{
			PostID:        id,
			Author:        body.Author,
			Content:       body.Content,
			ContentType:   body.ContentType,
			Hashtags:      body.Hashtags,
			ReplyTo:       body.ReplyTo,
			QuoteOf:       body.QuoteOf,
			CreatedAt:     body.CreatedAt,
			HasAttachment: len(body.Attachment) > 0,
		}
		if cfg.LikeCount != nil {
			item.Likes, _ = cfg.LikeCount(id)
		}
		item.Views, _ = cfg.Mailbox.ViewCount(id)
		out = append(out, item)
	}
	return out
}

// ── GET /feed/timeline ────────────────────────────────────────────────────

func feedTimeline(cfg FeedConfig) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		if r.Method != http.MethodGet {
			jsonErr(w, fmt.Errorf("method not allowed"), 405)
			return
		}
		follower := r.URL.Query().Get("follower")
		if follower == "" {
			jsonErr(w, fmt.Errorf("follower parameter required"), 400)
			return
		}
		if cfg.Following == nil || cfg.PostsByAuthor == nil {
			jsonErr(w, fmt.Errorf("timeline requires chain queries"), 503)
			return
		}
		limit := queryInt(r, "limit", 50)
		perAuthor := limit
		if perAuthor > 30 {
			perAuthor = 30
		}

		following, err := cfg.Following(follower)
		if err != nil {
			jsonErr(w, err, 500)
			return
		}
		var merged []*blockchain.PostRecord
		for _, target := range following {
			posts, err := cfg.PostsByAuthor(target, perAuthor)
			if err != nil {
				continue
			}
			for _, p := range posts {
				if p != nil && !p.Deleted {
					merged = append(merged, p)
				}
			}
		}
		// Sort newest-first, take top N.
		sort.Slice(merged, func(i, j int) bool { return merged[i].CreatedAt > merged[j].CreatedAt })
		if len(merged) > limit {
			merged = merged[:limit]
		}
		out := make([]feedAuthorItem, 0, len(merged))
		for _, rec := range merged {
			out = append(out, buildAuthorItem(cfg, rec))
		}
		jsonOK(w, map[string]any{"count": len(out), "posts": out})
	}
}

// ── GET /feed/trending ────────────────────────────────────────────────────

func feedTrending(cfg FeedConfig) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		if r.Method != http.MethodGet {
			jsonErr(w, fmt.Errorf("method not allowed"), 405)
			return
		}
		limit := queryInt(r, "limit", 30)
		// Window defaults to 24h; cap 7d so a viral post from a week ago
		// doesn't permanently dominate.
		windowHours := queryInt(r, "window", 24)
		if windowHours > 24*7 {
			windowHours = 24 * 7
		}
		if windowHours < 1 {
			windowHours = 1
		}
		ids, err := cfg.Mailbox.RecentPostIDs(int64(windowHours)*3600, 500)
		if err != nil {
			jsonErr(w, err, 500)
			return
		}
		// Score each = likes*3 + views, honoring soft-delete.
		type scored struct {
			id    string
			score uint64
		}
		scoredList := make([]scored, 0, len(ids))
		for _, id := range ids {
			if cfg.GetPost != nil {
				if rec, _ := cfg.GetPost(id); rec != nil && rec.Deleted {
					continue
				}
			}
			views, _ := cfg.Mailbox.ViewCount(id)
			var likes uint64
			if cfg.LikeCount != nil {
				likes, _ = cfg.LikeCount(id)
			}
			scoredList = append(scoredList, scored{id: id, score: likes*3 + views})
		}
		sort.Slice(scoredList, func(i, j int) bool { return scoredList[i].score > scoredList[j].score })
		if len(scoredList) > limit {
			scoredList = scoredList[:limit]
		}
		pickedIDs := make([]string, len(scoredList))
		for i, s := range scoredList {
			pickedIDs[i] = s.id
		}
		out := expandByID(cfg, pickedIDs)
		jsonOK(w, map[string]any{"count": len(out), "posts": out})
	}
}

// ── GET /feed/foryou ──────────────────────────────────────────────────────
//
// Simple recommendations heuristic for v2.0.0:
//   1. Compute the set of authors the user already follows.
//   2. Fetch recent posts from the relay (last 48h).
//   3. Filter OUT posts from followed authors (those live in /timeline).
//   4. Filter OUT posts the user has already liked.
//   5. Rank remaining by (likes × 3 + views) and return top N.
//
// Future improvements (tracked as v2.2.0 "Feed algorithm"):
//   - Weight by "followed-of-followed" signal (friends-of-friends boost).
//   - Decay by age (exp half-life ~12h).
//   - Penalise self-engagement (author liking own post).
//   - Collaborative filtering on hashtag co-occurrence.

func feedForYou(cfg FeedConfig) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		if r.Method != http.MethodGet {
			jsonErr(w, fmt.Errorf("method not allowed"), 405)
			return
		}
		pub := r.URL.Query().Get("pub")
		limit := queryInt(r, "limit", 30)

		// Gather user's follows + likes to exclude from the candidate pool.
		excludedAuthors := make(map[string]struct{})
		if cfg.Following != nil && pub != "" {
			if list, err := cfg.Following(pub); err == nil {
				for _, a := range list {
					excludedAuthors[a] = struct{}{}
				}
			}
		}
		// Post pool: last 48h on this relay.
		ids, err := cfg.Mailbox.RecentPostIDs(48*3600, 500)
		if err != nil {
			jsonErr(w, err, 500)
			return
		}
		type scored struct {
			id    string
			score uint64
		}
		scoredList := make([]scored, 0, len(ids))
		for _, id := range ids {
			body, _ := cfg.Mailbox.Get(id)
			if body == nil {
				continue
			}
			if _, followed := excludedAuthors[body.Author]; followed {
				continue
			}
			if body.Author == pub {
				continue // don't recommend user's own posts
			}
			if cfg.GetPost != nil {
				if rec, _ := cfg.GetPost(id); rec != nil && rec.Deleted {
					continue
				}
			}
			// Skip already-liked.
			if cfg.HasLiked != nil && pub != "" {
				if liked, _ := cfg.HasLiked(id, pub); liked {
					continue
				}
			}
			views, _ := cfg.Mailbox.ViewCount(id)
			var likes uint64
			if cfg.LikeCount != nil {
				likes, _ = cfg.LikeCount(id)
			}
			// Small "seed" score so posts with no engagement still get shown
			// sometimes (otherwise a silent but fresh post can't break in).
			scoredList = append(scoredList, scored{id: id, score: likes*3 + views + 1})
		}
		sort.Slice(scoredList, func(i, j int) bool { return scoredList[i].score > scoredList[j].score })
		if len(scoredList) > limit {
			scoredList = scoredList[:limit]
		}
		pickedIDs := make([]string, len(scoredList))
		for i, s := range scoredList {
			pickedIDs[i] = s.id
		}
		out := expandByID(cfg, pickedIDs)
		jsonOK(w, map[string]any{"count": len(out), "posts": out})
	}
}

// ── GET /feed/hashtag/{tag} ──────────────────────────────────────────────

func feedHashtag(cfg FeedConfig) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		if r.Method != http.MethodGet {
			jsonErr(w, fmt.Errorf("method not allowed"), 405)
			return
		}
		tag := strings.TrimPrefix(r.URL.Path, "/feed/hashtag/")
		tag = strings.Trim(tag, "/")
		if tag == "" {
			jsonErr(w, fmt.Errorf("tag required"), 400)
			return
		}
		limit := queryInt(r, "limit", 50)
		ids, err := cfg.Mailbox.PostsByHashtag(tag, limit)
		if err != nil {
			jsonErr(w, err, 500)
			return
		}
		out := expandByID(cfg, ids)
		jsonOK(w, map[string]any{"tag": strings.ToLower(tag), "count": len(out), "posts": out})
	}
}

// (queryInt helper is shared with the rest of the node HTTP surface;
// see api_common.go.)