feat(posts): in-context anchored feed with bidirectional infinite scroll

Provenance "View post" deep-links to /posts?post_id=X, which now opens the
feed centered on that post with infinite load in BOTH directions.

Backend: PostFeedService.scroll gains a direction (older|newer); new
around(post_id) returns a window of newer + the post + older with a cursor
for each end. /api/posts accepts ?around= and ?direction=. + API tests.

Frontend: posts store gains loadAround/loadOlder/loadNewer (older appends,
newer prepends) with per-end cursors; PostsView's anchored mode scrolls to
the post, observes top + bottom sentinels, and preserves scroll position on
upward prepend so the page doesn't jump. Normal feed mode unchanged.

Closes the remaining half of the post-navigation work.

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

backend/app/api/posts.py

@@ -18,6 +18,8 @@ async def list_posts():
     artist_id_raw = args.get("artist_id")
     platform = args.get("platform") or None
     limit_raw = args.get("limit", "24")
+    direction = args.get("direction", "older")
+    around_raw = args.get("around")
 
     try:
         limit = int(limit_raw)
@@ -26,6 +28,16 @@ async def list_posts():
     if limit < 1 or limit > 100:
         return _bad("invalid_limit", detail="limit must be between 1 and 100")
 
+    if direction not in ("older", "newer"):
+        return _bad("invalid_direction", detail="direction must be 'older' or 'newer'")
+
+    around_id = None
+    if around_raw is not None:
+        try:
+            around_id = int(around_raw)
+        except ValueError:
+            return _bad("invalid_around", detail="around must be an integer post id")
+
     artist_id = None
     if artist_id_raw is not None:
         try:
@@ -40,11 +52,20 @@ async def list_posts():
         )
 
     async with get_session() as session:
-        try:
-            page = await PostFeedService(session).scroll(
-                cursor=cursor, artist_id=artist_id,
+        svc = PostFeedService(session)
+        if around_id is not None:
+            result = await svc.around(
+                post_id=around_id, artist_id=artist_id,
                 platform=platform, limit=limit,
             )
+            if result is None:
+                return _bad("not_found", status=404, detail=f"post id={around_id}")
+            return jsonify(result)
+        try:
+            page = await svc.scroll(
+                cursor=cursor, artist_id=artist_id,
+                platform=platform, limit=limit, direction=direction,
+            )
         except ValueError as exc:
             # Service raises ValueError for malformed cursors only;
             # limit bounds are validated above.