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.

backend/app/services/post_feed_service.py

@@ -56,9 +56,17 @@ class PostFeedService:
         artist_id: int | None = None,
         platform: str | None = None,
         limit: int = 24,
+        direction: str = "older",
     ) -> dict:
+        """Paginate the feed from `cursor`. direction='older' walks back in
+        time (default, infinite-scroll down); direction='newer' walks forward
+        (scroll up in an anchored view). Items are always returned in feed
+        (descending) order; `next_cursor` points to the far edge in the
+        requested direction (null when exhausted)."""
         if limit < 1 or limit > 100:
             raise ValueError("limit must be between 1 and 100")
+        if direction not in ("older", "newer"):
+            raise ValueError("direction must be 'older' or 'newer'")
 
         sort_key = _sort_key()
         stmt = (
@@ -72,22 +80,37 @@ class PostFeedService:
             stmt = stmt.where(Source.platform == platform)
         if cursor:
             cur_ts, cur_id = decode_cursor(cursor)
-            stmt = stmt.where(
-                or_(
+            if direction == "older":
+                stmt = stmt.where(or_(
                     sort_key < cur_ts,
                     and_(sort_key == cur_ts, Post.id < cur_id),
-                )
-            )
+                ))
+            else:
+                stmt = stmt.where(or_(
+                    sort_key > cur_ts,
+                    and_(sort_key == cur_ts, Post.id > cur_id),
+                ))
 
-        stmt = stmt.order_by(sort_key.desc(), Post.id.desc()).limit(limit + 1)
+        if direction == "older":
+            stmt = stmt.order_by(sort_key.desc(), Post.id.desc())
+        else:
+            stmt = stmt.order_by(sort_key.asc(), Post.id.asc())
+        stmt = stmt.limit(limit + 1)
         rows = (await self.session.execute(stmt)).all()
 
+        has_more = len(rows) > limit
+        rows = rows[:limit]
+        if direction == "newer":
+            # Fetched ascending (closest-newer first); flip to feed order.
+            rows = list(reversed(rows))
+
         next_cursor: str | None = None
-        if len(rows) > limit:
-            last_post, _, _ = rows[limit - 1]
-            last_key = last_post.post_date or last_post.downloaded_at
-            next_cursor = encode_cursor(last_key, last_post.id)
-            rows = rows[:limit]
+        if has_more and rows:
+            # Far edge in the travel direction: oldest row going older,
+            # newest row going newer (rows is descending for display).
+            edge_post = rows[-1][0] if direction == "older" else rows[0][0]
+            edge_key = edge_post.post_date or edge_post.downloaded_at
+            next_cursor = encode_cursor(edge_key, edge_post.id)
 
         post_ids = [p.id for p, _, _ in rows]
         thumbs_map = await self._thumbnails_for(post_ids)
@@ -99,6 +122,49 @@ class PostFeedService:
         ]
         return {"items": items, "next_cursor": next_cursor}
 
+    async def around(
+        self,
+        *,
+        post_id: int,
+        artist_id: int | None = None,
+        platform: str | None = None,
+        limit: int = 12,
+    ) -> dict | None:
+        """A window centered on `post_id`: up to `limit` newer posts + the
+        post + up to `limit` older posts, in feed (descending) order, with a
+        cursor for each end. Returns None if the post doesn't exist."""
+        anchor = (await self.session.execute(
+            select(Post, Artist, Source)
+            .join(Source, Post.source_id == Source.id)
+            .join(Artist, Source.artist_id == Artist.id)
+            .where(Post.id == post_id)
+        )).one_or_none()
+        if anchor is None:
+            return None
+        anchor_post, anchor_artist, anchor_source = anchor
+        anchor_key = anchor_post.post_date or anchor_post.downloaded_at
+        anchor_cursor = encode_cursor(anchor_key, anchor_post.id)
+
+        older = await self.scroll(
+            cursor=anchor_cursor, artist_id=artist_id, platform=platform,
+            limit=limit, direction="older",
+        )
+        newer = await self.scroll(
+            cursor=anchor_cursor, artist_id=artist_id, platform=platform,
+            limit=limit, direction="newer",
+        )
+        thumbs_map = await self._thumbnails_for([anchor_post.id])
+        atts_map = await self._attachments_for([anchor_post.id])
+        anchor_item = self._to_dict(
+            anchor_post, anchor_artist, anchor_source, thumbs_map, atts_map,
+        )
+        return {
+            "items": newer["items"] + [anchor_item] + older["items"],
+            "cursor_older": older["next_cursor"],
+            "cursor_newer": newer["next_cursor"],
+            "anchor_id": anchor_post.id,
+        }
+
     async def get_post(self, post_id: int) -> dict | None:
         row = (await self.session.execute(
             select(Post, Artist, Source)