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/routers/feedback.py

@@ -34,7 +34,7 @@ class FeedbackResponse(BaseModel):
     feedback_id: str
     run_id: str
     thread_id: str
-    owner_id: str | None = None
+    user_id: str | None = None
     message_id: str | None = None
     rating: int
     comment: str | None = None
@@ -80,7 +80,7 @@ async def create_feedback(
         run_id=run_id,
         thread_id=thread_id,
         rating=body.rating,
-        owner_id=user_id,
+        user_id=user_id,
         message_id=body.message_id,
         comment=body.comment,
     )