feat(browse): sticky tabs + per-tab search bar (server-side, scope-aware)

The Browse tab nav scrolled away (operator didn't know it existed) and
Posts had no search. Roll the tab strip + a shared search field into one
sticky block pinned under the 64px TopNav.

- Posts gains server-side text search: PostFeedService.scroll()/around()
  + /api/posts accept q (ILIKE over post_title OR description), applied
  INSIDE the artist/platform WHERE so search stays scoped to the active
  filter. Scope shown as clearable chips next to the search field.
- Artists/Tags search consolidates into the sticky bar: their inner
  search boxes are removed; they react to route.query.q (q is deep-
  linkable, e.g. /browse?tab=posts&q=foo). Platform/kind filters stay.
- Posts empty state now distinguishes 'no matches' from 'no posts yet'.

Tests: posts q-search matches title|description and stays artist-scoped
(service); q passthrough (api).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api/posts.py

@@ -17,6 +17,7 @@ async def list_posts():
     cursor = args.get("cursor") or None
     artist_id_raw = args.get("artist_id")
     platform = args.get("platform") or None
+    q = (args.get("q") or "").strip() or None
     limit_raw = args.get("limit", "24")
     direction = args.get("direction", "older")
     around_raw = args.get("around")
@@ -56,7 +57,7 @@ async def list_posts():
         if around_id is not None:
             result = await svc.around(
                 post_id=around_id, artist_id=artist_id,
-                platform=platform, limit=limit,
+                platform=platform, q=q, limit=limit,
             )
             if result is None:
                 return _bad("not_found", status=404, detail=f"post id={around_id}")
@@ -64,7 +65,7 @@ async def list_posts():
         try:
             page = await svc.scroll(
                 cursor=cursor, artist_id=artist_id,
-                platform=platform, limit=limit, direction=direction,
+                platform=platform, q=q, limit=limit, direction=direction,
             )
         except ValueError as exc:
             # Service raises ValueError for malformed cursors only;

backend/app/services/post_feed_service.py

@@ -45,6 +45,7 @@ class PostFeedService:
         cursor: str | None = None,
         artist_id: int | None = None,
         platform: str | None = None,
+        q: str | None = None,
         limit: int = 24,
         direction: str = "older",
     ) -> dict:
@@ -52,7 +53,12 @@ class PostFeedService:
         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)."""
+        requested direction (null when exhausted).
+
+        `q` is a free-text filter (ILIKE substring over post_title OR
+        description) applied INSIDE the artist/platform scope, so a search
+        from the Browse bar stays within whatever artist is filtered in
+        view (operator-asked 2026-06-11)."""
         if limit < 1 or limit > 100:
             raise ValueError("limit must be between 1 and 100")
         if direction not in ("older", "newer"):
@@ -74,6 +80,12 @@ class PostFeedService:
             stmt = stmt.where(Post.artist_id == artist_id)
         if platform is not None:
             stmt = stmt.where(Source.platform == platform)
+        if q:
+            like = f"%{q}%"
+            stmt = stmt.where(or_(
+                Post.post_title.ilike(like),
+                Post.description.ilike(like),
+            ))
         if cursor:
             cur_ts, cur_id = decode_cursor(cursor)
             if direction == "older":
@@ -124,6 +136,7 @@ class PostFeedService:
         post_id: int,
         artist_id: int | None = None,
         platform: str | None = None,
+        q: str | None = None,
         limit: int = 12,
     ) -> dict | None:
         """A window centered on `post_id`: up to `limit` newer posts + the
@@ -143,11 +156,11 @@ class PostFeedService:
 
         older = await self.scroll(
             cursor=anchor_cursor, artist_id=artist_id, platform=platform,
-            limit=limit, direction="older",
+            q=q, limit=limit, direction="older",
         )
         newer = await self.scroll(
             cursor=anchor_cursor, artist_id=artist_id, platform=platform,
-            limit=limit, direction="newer",
+            q=q, limit=limit, direction="newer",
         )
         thumbs_map = await self._thumbnails_for([anchor_post.id])
         atts_map = await self._attachments_for([anchor_post.id])