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)

backend/storage/__init__.py

@@ -0,0 +1,27 @@
+"""
+Storage backend factory for DocuVault.
+
+Mirrors backend/ai/__init__.py — exposes a get_storage_backend() factory
+that returns the configured StorageBackend implementation.
+
+Phase 1 always returns MinIOBackend. Phase 5 will extend this factory to
+support OneDrive, Google Drive, Nextcloud, and WebDAV backends.
+"""
+from storage.base import StorageBackend
+from storage.minio_backend import MinIOBackend
+from config import settings
+
+
+def get_storage_backend() -> StorageBackend:
+    """Return a MinIOBackend instance configured from config.settings.
+
+    secure=False is correct for Docker internal HTTP traffic between containers
+    (RESEARCH.md Pattern 3).
+    """
+    return MinIOBackend(
+        endpoint=settings.minio_endpoint,
+        access_key=settings.minio_access_key,
+        secret_key=settings.minio_secret_key,
+        bucket=settings.minio_bucket,
+        secure=False,
+    )