feat(feed): relay body storage + HTTP endpoints (Phase B of v2.0.0)

Phase A (the previous commit) added the on-chain foundations. Phase B
is the off-chain layer: post bodies live in a BadgerDB-backed feed
mailbox, and a full HTTP surface makes the feed usable from clients.

New components

  relay/feed_mailbox.go (+ tests)
    - FeedPost: body + content-type + attachment + hashtags + thread refs
    - Store / Get / Delete with TTL-bounded eviction (30 days default)
    - View counter (IncrementView / ViewCount) — off-chain because one
      tx per view would be nonsense
    - Hashtag inverted index: auto-extracts #tokens from content on
      Store, lowercased + deduped + capped at 8/post
    - Author chrono index: PostsByAuthor returns newest-first IDs
    - RecentPostIDs: scan-by-age helper used by trending/foryou

  node/api_feed.go
    POST /feed/publish           — author-signed body upload, returns
                                   post_id + content_hash + size +
                                   hashtags + estimated fee for the
                                   follow-up on-chain CREATE_POST tx
    GET  /feed/post/{id}         — fetch body (respects on-chain soft
                                   delete, returns 410 when deleted)
    GET  /feed/post/{id}/stats   — {views, likes, liked_by_me?}
    POST /feed/post/{id}/view    — bump the counter
    GET  /feed/author/{pub}      — chain-authoritative post list
                                   enriched with body + stats
    GET  /feed/timeline          — merged feed from people the user
                                   follows (reads chain.Following,
                                   fetches each author's recent posts)
    GET  /feed/trending          — top-scored posts in last 24h
                                   (score = likes × 3 + views)
    GET  /feed/foryou            — simple recommendations: recent posts
                                   minus authors the user already
                                   follows, already-liked posts, and
                                   own posts; ranked by engagement
    GET  /feed/hashtag/{tag}     — posts tagged with the given #tag

  cmd/node/main.go wiring
    - --feed-db flag (DCHAIN_FEED_DB) + --feed-ttl-days (DCHAIN_FEED_TTL_DAYS)
    - Opens FeedMailbox + registers FeedRoutes alongside RelayRoutes
    - Threads chain.Post / LikeCount / HasLiked / PostsByAuthor / Following
      into FeedConfig so HTTP handlers can merge on-chain metadata with
      off-chain body+stats.

Auth & safety
  - POST /feed/publish: Ed25519 signature over "publish:<post_id>:
    <content_sha256_hex>:<ts>"; ±5-minute skew window for anti-replay.
  - content_hash binds body to the on-chain tx — you can't publish
    body-A off-chain and commit hash-of-body-B on-chain.
  - Writes wrapped in withSubmitTxGuards (rate-limit + size cap), reads
    in withReadLimit — same guards as /relay.

Trending / recommendations
  - V1 heuristic (likes × 3 + views) + time window. Documented as
    v2.2.0 "Feed algorithm" candidate for a proper ranking layer
    (half-life decay, follow-of-follow boost, hashtag collaborative).

Tests
  - Store round-trip, size enforcement, hashtag indexing (case-insensitive
    + dedup), view counter increments, author chrono order, delete
    cleans all indices, RecentPostIDs time-window filter.
  - Full go test ./... is green (blockchain + consensus + identity +
    relay + vm all pass).

Next (Phase C): client Feed tab — composer, timeline, post detail,
profile follow, For You + Trending screens.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/node/main.go

@@ -77,6 +77,8 @@ func main() {
 	registerRelay    := flag.Bool("register-relay", envBoolOr("DCHAIN_REGISTER_RELAY", false), "submit REGISTER_RELAY tx on startup (env: DCHAIN_REGISTER_RELAY)")
 	relayFee         := flag.Uint64("relay-fee", envUint64Or("DCHAIN_RELAY_FEE", 1_000), "relay fee per message in µT (env: DCHAIN_RELAY_FEE)")
 	mailboxDB        := flag.String("mailbox-db", envOr("DCHAIN_MAILBOX_DB", "./mailboxdata"), "BadgerDB directory for relay mailbox (env: DCHAIN_MAILBOX_DB)")
+	feedDB           := flag.String("feed-db", envOr("DCHAIN_FEED_DB", "./feeddata"), "BadgerDB directory for social-feed post bodies (env: DCHAIN_FEED_DB)")
+	feedTTLDays      := flag.Int("feed-ttl-days", int(envUint64Or("DCHAIN_FEED_TTL_DAYS", 30)), "how long feed posts are retained before auto-eviction (env: DCHAIN_FEED_TTL_DAYS)")
 	govContractID    := flag.String("governance-contract", envOr("DCHAIN_GOVERNANCE_CONTRACT", ""), "governance contract ID for dynamic chain parameters (env: DCHAIN_GOVERNANCE_CONTRACT)")
 	joinSeedURL      := flag.String("join", envOr("DCHAIN_JOIN", ""), "bootstrap from a running node: comma-separated HTTP URLs (env: DCHAIN_JOIN)")
 	// Observer mode: the node participates in the P2P network, applies
@@ -634,6 +636,15 @@ func main() {
 	go mailbox.RunGC()
 	log.Printf("[NODE] relay mailbox: %s", *mailboxDB)
 
+	// --- Feed mailbox (social-feed post bodies, v2.0.0) ---
+	feedTTL := time.Duration(*feedTTLDays) * 24 * time.Hour
+	feedMailbox, err := relay.OpenFeedMailbox(*feedDB, feedTTL)
+	if err != nil {
+		log.Fatalf("[NODE] feed mailbox: %v", err)
+	}
+	defer feedMailbox.Close()
+	log.Printf("[NODE] feed mailbox: %s (TTL %d days)", *feedDB, *feedTTLDays)
+
 	// Push-notify bus consumers whenever a fresh envelope lands in the
 	// mailbox. Clients subscribed to `inbox:<my_x25519>` (via WS) get the
 	// event immediately so they no longer need to poll /relay/inbox.
@@ -927,6 +938,16 @@ func main() {
 		},
 	}
 
+	feedConfig := node.FeedConfig{
+		Mailbox:         feedMailbox,
+		HostingRelayPub: id.PubKeyHex(),
+		GetPost:         chain.Post,
+		LikeCount:       chain.LikeCount,
+		HasLiked:        chain.HasLiked,
+		PostsByAuthor:   chain.PostsByAuthor,
+		Following:       chain.Following,
+	}
+
 	go func() {
 		log.Printf("[NODE] stats API:   http://0.0.0.0%s/stats", *statsAddr)
 		if *disableUI {
@@ -947,6 +968,7 @@ func main() {
 		if err := stats.ListenAndServe(*statsAddr, statsQuery, func(mux *http.ServeMux) {
 			node.RegisterExplorerRoutes(mux, explorerQuery, routeFlags)
 			node.RegisterRelayRoutes(mux, relayConfig)
+			node.RegisterFeedRoutes(mux, feedConfig)
 
 			// POST /api/governance/link — link deployed contracts at runtime.
 			// Body: {"governance": "<id>"}