test(03-01): add Wave 0 xfail stubs and shared fixtures for Phase 3

- Add auth_user, admin_user, mock_minio_presigned, mock_minio_stat fixtures to conftest.py
- Create test_quota.py with 4 xfail stubs (STORE-03, STORE-05, STORE-06, SC2 race)
- Append test_migration_0003 to test_alembic.py (full pre-seed + post-migration assertions)
- Append 3 classifier xfail stubs (DOC-03, DOC-05, D-15)
- Append 6 document xfail stubs (D-05, STORE-04, SEC-04, D-16)
- Append 4 topic xfail stubs (DOC-04, D-09, D-17)
- Append test_settings_endpoint_removed stub (D-12)
- All 19 new test IDs collect cleanly with xfail(strict=False)

backend/tests/conftest.py

@@ -176,3 +176,129 @@ def sample_pdf(tmp_path):
     doc.save(str(pdf_path))
     doc.close()
     return pdf_path
+
+
+# ── Phase 3 shared fixtures ───────────────────────────────────────────────────
+# These fixtures are used by test_quota.py, test_documents.py, test_topics.py,
+# and test_classifier.py in Plans 03-02 through 03-04.
+
+
+@pytest_asyncio.fixture
+async def auth_user(db_session: AsyncSession):
+    """Create a regular user with a Quota row and return auth context.
+
+    Returns dict with keys:
+      - user: User ORM instance
+      - token: signed JWT access token
+      - headers: {"Authorization": "Bearer <token>"}
+
+    The fixture issues a valid access token via services.auth.create_access_token
+    so that get_current_user accepts it in downstream endpoint tests.
+    """
+    import uuid as _uuid
+    from db.models import User, Quota
+    from services.auth import hash_password, create_access_token
+
+    user_id = _uuid.uuid4()
+    user = User(
+        id=user_id,
+        handle=f"testuser_{user_id.hex[:8]}",
+        email=f"testuser_{user_id.hex[:8]}@example.com",
+        password_hash=hash_password("Testpassword123!"),
+        role="user",
+        is_active=True,
+        password_must_change=False,
+    )
+    quota = Quota(
+        user_id=user_id,
+        limit_bytes=104857600,  # 100 MB
+        used_bytes=0,
+    )
+    db_session.add(user)
+    db_session.add(quota)
+    await db_session.commit()
+
+    token = create_access_token(str(user_id), "user")
+    return {
+        "user": user,
+        "token": token,
+        "headers": {"Authorization": f"Bearer {token}"},
+    }
+
+
+@pytest_asyncio.fixture
+async def admin_user(db_session: AsyncSession):
+    """Create an admin user with a Quota row and return auth context.
+
+    Returns the same dict shape as auth_user but with role="admin".
+    """
+    import uuid as _uuid
+    from db.models import User, Quota
+    from services.auth import hash_password, create_access_token
+
+    user_id = _uuid.uuid4()
+    user = User(
+        id=user_id,
+        handle=f"adminuser_{user_id.hex[:8]}",
+        email=f"adminuser_{user_id.hex[:8]}@example.com",
+        password_hash=hash_password("Testpassword123!"),
+        role="admin",
+        is_active=True,
+        password_must_change=False,
+    )
+    quota = Quota(
+        user_id=user_id,
+        limit_bytes=104857600,
+        used_bytes=0,
+    )
+    db_session.add(user)
+    db_session.add(quota)
+    await db_session.commit()
+
+    token = create_access_token(str(user_id), "admin")
+    return {
+        "user": user,
+        "token": token,
+        "headers": {"Authorization": f"Bearer {token}"},
+    }
+
+
+@pytest.fixture
+def mock_minio_presigned(monkeypatch):
+    """Patch MinIOBackend.generate_presigned_put_url with an AsyncMock.
+
+    The patched method does not exist yet — it is added in Plan 03-02.
+    Using raising=False ensures the patch installs before the attribute exists.
+
+    Yields the AsyncMock so tests can assert call counts and args.
+    """
+    from unittest.mock import AsyncMock
+
+    mock = AsyncMock(return_value="http://localhost:9000/docuvault/test-presigned-url")
+    try:
+        from storage.minio_backend import MinIOBackend
+        monkeypatch.setattr(MinIOBackend, "generate_presigned_put_url", mock, raising=False)
+    except ImportError:
+        pass  # storage module not yet available — patch is best-effort
+    yield mock
+
+
+@pytest.fixture
+def mock_minio_stat(monkeypatch):
+    """Patch MinIOBackend.stat_object with an AsyncMock returning 1024 bytes.
+
+    The patched method does not exist yet — it is added in Plan 03-02.
+    Using raising=False ensures the patch installs before the attribute exists.
+
+    Yields the AsyncMock for per-test customization:
+        mock_minio_stat.return_value = 50_000_000
+    """
+    from unittest.mock import AsyncMock
+
+    mock = AsyncMock(return_value=1024)
+    try:
+        from storage.minio_backend import MinIOBackend
+        monkeypatch.setattr(MinIOBackend, "stat_object", mock, raising=False)
+    except ImportError:
+        pass  # storage module not yet available — patch is best-effort
+    yield mock