feat(storage): enhance storage package with async provider and docs

Add to packages/storage/:
- README.md and README_zh.md - comprehensive documentation
- store/persistence/async_provider.py - async persistence provider

Update repositories:
- contracts/thread_meta.py - add new contract method
- db/thread_meta.py - implement new method
- factory.py - update factory logic

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/packages/storage/store/repositories/contracts/thread_meta.py

@@ -39,6 +39,7 @@ class ThreadMetaRepositoryProtocol(Protocol):
         self,
         thread_id: str,
         *,
+        assistant_id: str | None = None,
         display_name: str | None = None,
         status: str | None = None,
         metadata: dict[str, Any] | None = None,