curo1305
eaf86a832a
feat(01-04): add StorageBackend ABC + MinIOBackend + factory
- backend/storage/base.py: StorageBackend ABC with 5 abstract methods mirroring ai/base.py
- backend/storage/minio_backend.py: MinIOBackend wrapping all sync Minio SDK calls in asyncio.to_thread(); STORE-02 key schema: {user_id}/{document_id}/{uuid4()}{ext}
- backend/storage/__init__.py: get_storage_backend() factory mirroring ai/__init__.py
- backend/tests/test_storage.py: remove xfail markers (plan 04 implements the module)