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/test_settings.py

@@ -104,3 +104,20 @@ async def test_get_default_prompt(async_client):
     assert resp.status_code == 200
     assert "system_prompt" in resp.json()
     assert len(resp.json()["system_prompt"]) > 0
+
+
+# ---------------------------------------------------------------------------
+# Wave 0 xfail stub — D-12: /api/settings endpoint removed in Plan 03-04
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.xfail(strict=False, reason="implemented in plan 03-04")
+async def test_settings_endpoint_removed(async_client):
+    """GET /api/settings returns 404 after the flat-file settings system is retired.
+
+    D-12: the /api/settings endpoint is removed entirely in Phase 3. All AI config
+    comes from the database (users.ai_provider / users.ai_model set by admin).
+    The flat-file services/storage.py load_settings()/save_settings() functions
+    are also deleted (CONTEXT.md D-12).
+    """
+    assert True  # scaffold