feat: brain masonry card grid — Karakeep-style layout with Atelier aesthetics

- 3-column CSS masonry grid (2 on tablet, 1 on mobile)
- Link cards show screenshot thumbnails
- Note cards show content body inline
- Tags as pills, folder/date in meta footer
- Screenshot serving endpoint added to brain API
- Auto-polling for pending items (3s interval)
- Detail sheet shows raw_content for notes
- Warm frosted glass card styling matching Atelier design

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

services/brain/app/api/routes.py

@@ -19,6 +19,7 @@ from app.models.schema import (
     HybridSearchQuery, SearchResult, ConfigOut,
 )
 from app.services.storage import storage
+from fastapi.responses import Response
 from app.worker.tasks import enqueue_process_item
 
 router = APIRouter(prefix="/api", tags=["brain"])
@@ -317,3 +318,22 @@ async def hybrid_search(
         folder=body.folder, tags=body.tags, item_type=body.type, limit=body.limit,
     )
     return SearchResult(items=items, total=len(items), query=body.q)
+
+
+# ── Serve stored files (screenshots, archived HTML) ──
+
+@router.get("/storage/{item_id}/{asset_type}/{filename}")
+async def serve_asset(item_id: str, asset_type: str, filename: str):
+    """Serve a stored asset file."""
+    path = f"{item_id}/{asset_type}/{filename}"
+    if not storage.exists(path):
+        raise HTTPException(status_code=404, detail="Asset not found")
+
+    data = storage.read(path)
+    ct = "application/octet-stream"
+    if filename.endswith(".png"): ct = "image/png"
+    elif filename.endswith(".jpg") or filename.endswith(".jpeg"): ct = "image/jpeg"
+    elif filename.endswith(".html"): ct = "text/html"
+    elif filename.endswith(".pdf"): ct = "application/pdf"
+
+    return Response(content=data, media_type=ct, headers={"Cache-Control": "public, max-age=3600"})