feat: TDD Phase 1 — 44 backend API tests passing

Test infrastructure:
- conftest.py with SQLite in-memory DB, isolated per-test cleanup
- pytest.ini with asyncio auto mode
- Fixtures: admin/dba/developer/guest/disabled users, tokens, test instance

Test coverage (44 tests):
- test_auth: login/logout/me/token expiry (8 tests)
- test_users: CRUD + role permissions (8 tests)
- test_instances: CRUD + connection test mock (8 tests)
- test_tickets: CRUD + review/execute workflow + pagination (11 tests)
- test_query: select/drop/delete rejection + guest deny (4 tests)
- test_dashboard: stats + auth check (2 tests)

Backend fix:
- query endpoint now catches ValueError from QueryService → 400 instead of 500

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/pytest.ini

@@ -0,0 +1,2 @@
+[pytest]
+asyncio_mode = auto