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/app/api/v1/endpoints/query.py

@@ -27,7 +27,10 @@ async def execute_query(
     if not instance:
         raise HTTPException(status_code=404, detail="数据源不存在")
 
-    query_result = await QueryService.execute_query(instance, req.database, req.sql, current_user.id)
+    try:
+        query_result = await QueryService.execute_query(instance, req.database, req.sql, current_user.id)
+    except ValueError as e:
+        raise HTTPException(status_code=400, detail=str(e))
 
     # Record audit log
     log = AuditLog(