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/rbac.py

@@ -1,16 +1,18 @@
 import json
 import os
-import threading
 import tempfile
-from typing import Optional
+import threading
+
+from fastapi import HTTPException, Request
 from pydantic import BaseModel
-from fastapi import HTTPException, Request, status, Depends
 
 import config
 
+
 def get_rbac_file():
     return os.path.join(config.VOLUME_ROOT, "docker/nas-dashboard/rbac.json")
 
+
 _rbac_lock = threading.RLock()
 
 ROLE_GROUP_MAP = {
@@ -75,7 +77,7 @@ def save_rbac(data: dict):
     _atomic_json_write(get_rbac_file(), data)
 
 
-def resolve_role(groups: list[str]) -> Optional[str]:
+def resolve_role(groups: list[str]) -> str | None:
     for group in groups:
         if group in ROLE_GROUP_MAP:
             return ROLE_GROUP_MAP[group]
@@ -107,6 +109,7 @@ def get_sidebar_order(username: str) -> dict | None:
 def save_sidebar_order(username: str, order: dict):
     def mutator(rbac):
         rbac.setdefault("user_overrides", {}).setdefault(username, {})["sidebar_order"] = order
+
     update_rbac(mutator)
 
 
@@ -118,26 +121,32 @@ def has_page_access(user: User, page: str) -> bool:
 
 def require_page(page: str):
     """FastAPI dependency factory — 403 if user lacks page access."""
+
     async def checker(request: Request):
         user: User = request.state.user
         if not has_page_access(user, page):
             raise HTTPException(status_code=403, detail="Access denied")
+
     return checker
 
 
 def require_admin():
     """FastAPI dependency — 403 if user is not admin."""
+
     async def checker(request: Request):
         user: User = request.state.user
         if user.role != "admin":
             raise HTTPException(status_code=403, detail="Admin only")
+
     return checker
 
 
 def require_write():
     """FastAPI dependency — 403 if viewer tries non-GET method."""
+
     async def checker(request: Request):
         user: User = request.state.user
         if user.role == "viewer" and request.method != "GET":
             raise HTTPException(status_code=403, detail="Read-only access")
+
     return checker