refactor(persistence): rename owner_id to user_id and thread_meta_repo to thread_store

Rename owner_id to user_id across all persistence models, repositories,
stores, routers, and tests for clearer semantics. Rename thread_meta_repo
to thread_store for consistency with run_store/run_event_store naming.
Add ThreadMetaStore return type annotation to get_thread_store().

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

backend/app/gateway/langgraph_auth.py

@@ -93,14 +93,14 @@ async def authenticate(request):
 
 @auth.on
 async def add_owner_filter(ctx: Auth.types.AuthContext, value: dict):
-    """Inject owner_id metadata on writes; filter by owner_id on reads.
+    """Inject user_id metadata on writes; filter by user_id on reads.
 
-    Gateway stores thread ownership as ``metadata.owner_id``.
+    Gateway stores thread ownership as ``metadata.user_id``.
     This handler ensures LangGraph Server enforces the same isolation.
     """
-    # On create/update: stamp owner_id into metadata
+    # On create/update: stamp user_id into metadata
     metadata = value.setdefault("metadata", {})
-    metadata["owner_id"] = ctx.user.identity
+    metadata["user_id"] = ctx.user.identity
 
     # Return filter dict — LangGraph applies it to search/read/delete
-    return {"owner_id": ctx.user.identity}
+    return {"user_id": ctx.user.identity}