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/tests/test_storage.py

@@ -20,7 +20,6 @@ import pytest
 # Test 1: object key matches STORE-02 regex
 # ---------------------------------------------------------------------------
 
-@pytest.mark.xfail(strict=False, reason="implemented in plan 04")
 async def test_object_key_schema(db_session):
     """STORE-02: put_object must return a key matching {user_id}/{doc_id}/{uuid4}{ext}."""
     try:
@@ -69,7 +68,6 @@ async def test_object_key_schema(db_session):
 # Test 2: human filename never in object key
 # ---------------------------------------------------------------------------
 
-@pytest.mark.xfail(strict=False, reason="implemented in plan 04")
 async def test_filename_not_in_object_key():
     """STORE-02: The human-readable filename must never appear in the MinIO object key."""
     try:
@@ -102,7 +100,6 @@ async def test_filename_not_in_object_key():
 # Test 3: StorageBackend ABC enforcement
 # ---------------------------------------------------------------------------
 
-@pytest.mark.xfail(strict=False, reason="implemented in plan 04")
 def test_storage_backend_abc_methods():
     """StorageBackend is abstract — concrete subclass missing all 5 methods raises TypeError."""
     try:
@@ -121,7 +118,6 @@ def test_storage_backend_abc_methods():
 # Test 4: factory returns MinIOBackend instance
 # ---------------------------------------------------------------------------
 
-@pytest.mark.xfail(strict=False, reason="implemented in plan 04")
 def test_get_storage_backend_returns_minio():
     """get_storage_backend() factory must return a MinIOBackend instance."""
     try:
@@ -138,7 +134,6 @@ def test_get_storage_backend_returns_minio():
 # Test 5: put_object wraps sync SDK call in asyncio.to_thread
 # ---------------------------------------------------------------------------
 
-@pytest.mark.xfail(strict=False, reason="implemented in plan 04")
 async def test_put_object_uses_asyncio_to_thread(monkeypatch):
     """MinIOBackend.put_object must delegate the blocking SDK call via asyncio.to_thread."""
     try:
@@ -182,7 +177,6 @@ async def test_put_object_uses_asyncio_to_thread(monkeypatch):
 # Test 6: health_check returns bool
 # ---------------------------------------------------------------------------
 
-@pytest.mark.xfail(strict=False, reason="implemented in plan 04")
 async def test_minio_backend_health_check_returns_bool():
     """MinIOBackend.health_check() returns True when bucket exists, False on exception."""
     try: