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/files.py

@@ -2,8 +2,10 @@ import os
 import re
 import shutil
 from pathlib import Path
-from fastapi import APIRouter, Depends, UploadFile, File, HTTPException
+
+from fastapi import APIRouter, Depends, File, HTTPException, UploadFile
 from fastapi.responses import FileResponse
+
 from config import VOLUME_ROOT
 from rbac import require_admin
 
@@ -49,7 +51,7 @@ def _is_safe_symlink(item: Path) -> bool:
 def _sanitize_filename(name: str) -> str:
     name = os.path.basename(name)
     name = name.replace("\x00", "").replace("/", "").replace("\\", "")
-    name = re.sub(r'\.\.+', '.', name)
+    name = re.sub(r"\.\.+", ".", name)
     name = name.strip(". ")
     if not name:
         raise ValueError("invalid filename")