feat(feed/chat): lazy-render + pagination for long scrolls

Server pagination
  - blockchain.PostsByAuthor signature extended with beforeTs int64;
    passing 0 keeps the previous "everything, newest first" behaviour,
    non-zero skips posts with CreatedAt >= beforeTs so clients can
    paginate older results.
  - node.FeedConfig.PostsByAuthor callback type updated; the two
    /feed endpoints that use it (timeline + author) now accept
    `?before=<unix_seconds>` and forward it through. /feed/author
    limit default dropped from 50 to 30 to match the client's page
    size.
  - node/api_common.go: new queryInt64 helper for parsing the cursor
    param safely (matches the queryInt pattern already used).

Client infinite scroll (Feed tab)
  - lib/feed.ts: fetchTimeline / fetchAuthorPosts accept
    `{limit?, before?}` options. Old signatures still work for other
    callers (fetchForYou / fetchTrending / fetchHashtag) — those are
    ranked feeds that don't have a stable cursor so they stay
    single-shot.
  - feed/index.tsx: tracks loadingMore / exhausted state. onEndReached
    (threshold 0.6) fires loadMore() which fetches the next 20 posts
    using the oldest currently-loaded post's created_at as `before`.
    Deduplicates on post_id before appending. Stops when the server
    returns < PAGE_SIZE items. ListFooterComponent shows a small
    spinner during paginated fetches.
  - FlatList lazy-render tuning on all feed lists (index + hashtag):
    initialNumToRender:10, maxToRenderPerBatch:8, windowSize:7,
    removeClippedSubviews — first paint stays quick even with 100+
    posts loaded.

Chat lazy render
  - chats/[id].tsx FlatList: initialNumToRender:25 (~1.5 screens),
    maxToRenderPerBatch:12, windowSize:10, removeClippedSubviews.
    Keeps initial chat open snappy on conversations with thousands
    of messages; RN re-renders a small window around the viewport
    and drops the rest.

Tests
  - chain_test.go updated for new PostsByAuthor signature.
  - All 7 Go packages green.
  - tsc --noEmit clean.

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

node/api_common.go

@@ -38,6 +38,20 @@ func queryInt(r *http.Request, key string, def int) int {
 	return n
 }
 
+// queryInt64 reads a non-negative int64 query param — typically a unix
+// timestamp cursor for pagination. Returns def when missing or invalid.
+func queryInt64(r *http.Request, key string, def int64) int64 {
+	s := r.URL.Query().Get(key)
+	if s == "" {
+		return def
+	}
+	n, err := strconv.ParseInt(s, 10, 64)
+	if err != nil || n < 0 {
+		return def
+	}
+	return n
+}
+
 // queryIntMin0 parses a query param as a non-negative integer; returns 0 if absent or invalid.
 func queryIntMin0(r *http.Request, key string) int {
 	s := r.URL.Query().Get(key)

node/api_feed.go

@@ -72,7 +72,7 @@ type FeedConfig struct {
 	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)
+	PostsByAuthor   func(authorPub string, beforeTs int64, limit int) ([]*blockchain.PostRecord, error)
 	Following       func(followerPub string) ([]string, error)
 }
 
@@ -440,13 +440,14 @@ func feedAuthor(cfg FeedConfig) http.HandlerFunc {
 			jsonErr(w, fmt.Errorf("author pub required"), 400)
 			return
 		}
-		limit := queryInt(r, "limit", 50)
+		limit := queryInt(r, "limit", 30)
+		beforeTs := queryInt64(r, "before", 0) // pagination cursor (unix seconds)
 
 		// 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)
+			records, err := cfg.PostsByAuthor(pub, beforeTs, limit)
 			if err != nil {
 				jsonErr(w, err, 500)
 				return
@@ -461,6 +462,8 @@ func feedAuthor(cfg FeedConfig) http.HandlerFunc {
 			jsonOK(w, map[string]any{"author": pub, "count": len(out), "posts": out})
 			return
 		}
+		// Fallback: relay index (no chain). Doesn't support `before` yet;
+		// the chain-authoritative path above is what production serves.
 		ids, err := cfg.Mailbox.PostsByAuthor(pub, limit)
 		if err != nil {
 			jsonErr(w, err, 500)
@@ -556,7 +559,8 @@ func feedTimeline(cfg FeedConfig) http.HandlerFunc {
 			jsonErr(w, fmt.Errorf("timeline requires chain queries"), 503)
 			return
 		}
-		limit := queryInt(r, "limit", 50)
+		limit := queryInt(r, "limit", 30)
+		beforeTs := queryInt64(r, "before", 0)  // pagination cursor
 		perAuthor := limit
 		if perAuthor > 30 {
 			perAuthor = 30
@@ -569,7 +573,7 @@ func feedTimeline(cfg FeedConfig) http.HandlerFunc {
 		}
 		var merged []*blockchain.PostRecord
 		for _, target := range following {
-			posts, err := cfg.PostsByAuthor(target, perAuthor)
+			posts, err := cfg.PostsByAuthor(target, beforeTs, perAuthor)
 			if err != nil {
 				continue
 			}