fix(threads): fall back to Store search when ThreadMetaRepository is unavailable

When database.backend=memory (default) or no SQL session factory is
configured, search_threads now queries the LangGraph Store instead of
returning 503. Returns empty list if neither Store nor repo is available.

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

backend/app/gateway/routers/threads.py

@@ -332,22 +332,15 @@ async def create_thread(body: ThreadCreateRequest, request: Request) -> ThreadRe
 
 @router.post("/search", response_model=list[ThreadResponse])
 async def search_threads(body: ThreadSearchRequest, request: Request) -> list[ThreadResponse]:
-    """Search and list threads from the threads_meta table.
+    """Search and list threads.
 
-    NOTE: Migration from pre-persistence-layer deployments:
+    Uses ThreadMetaRepository (SQL) when available, otherwise falls back
-    Threads created via LangGraph Server before this change are NOT
+    to the LangGraph Store for memory/lightweight deployments.
-    automatically indexed in threads_meta. They will not appear in
-    search results until a new run is created on them (which triggers
-    thread_meta upsert in services.py). For bulk migration, run:
-      python -m deerflow.persistence.migrate_threads_from_checkpointer
-    (migration script TBD in a follow-up PR)
     """
     from app.gateway.deps import get_thread_meta_repo
 
     repo = get_thread_meta_repo(request)
-    if repo is None:
+    if repo is not None:
-        raise HTTPException(status_code=503, detail="Thread metadata store not available")
-
         rows = await repo.search(
             metadata=body.metadata or None,
             status=body.status,
@@ -367,6 +360,36 @@ async def search_threads(body: ThreadSearchRequest, request: Request) -> list[Th
             for r in rows
         ]
 
+    # Fallback: search the LangGraph Store (memory / no-SQL deployments)
+    store = get_store(request)
+    if store is None:
+        return []
+
+    filter_dict: dict[str, Any] = {}
+    if body.metadata:
+        filter_dict.update(body.metadata)
+    if body.status:
+        filter_dict["status"] = body.status
+
+    items = await store.asearch(
+        THREADS_NS,
+        filter=filter_dict or None,
+        limit=body.limit,
+        offset=body.offset,
+    )
+    return [
+        ThreadResponse(
+            thread_id=item.key,
+            status=item.value.get("status", "idle"),
+            created_at=str(item.value.get("created_at", "")),
+            updated_at=str(item.value.get("updated_at", "")),
+            metadata=item.value.get("metadata", {}),
+            values=item.value.get("values", {}),
+            interrupts={},
+        )
+        for item in items
+    ]
+
 
 @router.patch("/{thread_id}", response_model=ThreadResponse)
 async def patch_thread(thread_id: str, body: ThreadPatchRequest, request: Request) -> ThreadResponse: