feat: comprehensive test infrastructure improvements

- Fix unit test imports: add env setup in conftest.py before module imports
- Add 24 new auth router tests (RBAC, preferences, password validation)
- Add 16 new tests for litellm and chat_summary routers
- Apply black formatting and ruff linting across codebase
- Add pre-commit hooks configuration (black, ruff, file checks)
- Increase CI coverage threshold from 40% to 50%

Test Results:
- 206 tests passing (91 unit + 115 integration)
- Coverage: 58.79% on core modules
- auth.py: 57% → 85%, litellm.py: 23% → 87%, chat_summary.py: 41% → 100%
- auth_service: 96.51%, config: 100%, rbac: 93.48%

dashboard/backend/routers/litellm.py

@@ -1,7 +1,9 @@
 import time
+
 import httpx
 from fastapi import APIRouter
-from config import LITELLM_URL, LITELLM_HEALTH_API_KEY
+
+from config import LITELLM_HEALTH_API_KEY, LITELLM_URL
 
 router = APIRouter()