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/base.py

@@ -0,0 +1,54 @@
+"""
+StorageBackend ABC for DocuVault.
+
+Mirrors backend/ai/base.py — declares the abstract interface that all storage
+backends (MinIO, OneDrive, Google Drive, Nextcloud, WebDAV) must implement.
+
+Five abstract methods define the contract:
+  put_object     — store bytes, return object key
+  get_object     — fetch bytes by key
+  delete_object  — remove object by key
+  presigned_get_url — generate a time-limited download URL
+  health_check   — verify backend connectivity
+"""
+from abc import ABC, abstractmethod
+
+
+class StorageBackend(ABC):
+    """Abstract base class for DocuVault object storage backends."""
+
+    @abstractmethod
+    async def put_object(
+        self,
+        user_id: str,
+        document_id: str,
+        file_bytes: bytes,
+        extension: str,
+        content_type: str,
+    ) -> str:
+        """Store bytes and return the generated object key.
+
+        The key MUST follow the STORE-02 schema: {user_id}/{document_id}/{uuid4()}{ext}.
+        The human-readable filename MUST NOT appear in the returned key.
+        """
+        ...
+
+    @abstractmethod
+    async def get_object(self, object_key: str) -> bytes:
+        """Fetch object bytes by key. Raises on missing key."""
+        ...
+
+    @abstractmethod
+    async def delete_object(self, object_key: str) -> None:
+        """Delete an object by key. No-op if the key does not exist."""
+        ...
+
+    @abstractmethod
+    async def presigned_get_url(self, object_key: str, expires_minutes: int = 60) -> str:
+        """Return a time-limited pre-signed download URL for the object."""
+        ...
+
+    @abstractmethod
+    async def health_check(self) -> bool:
+        """Return True if the backend is reachable and operational."""
+        ...