fix: resolve all CI lint and security issues

- ruff: 602→0 errors (428 auto-fixed, pyproject.toml ignores for FastAPI patterns)
- bandit: skip B310 (urllib for vault file access is intentional)
- Fixed SIM118 (dict.keys()→dict), PERF102, SIM113, SIM117
- Created pyproject.toml with ruff + bandit config
- 285 tests still pass

backend/auth/user_store.py

@@ -4,12 +4,11 @@
 # via tmp+rename to prevent corruption on crash.
 
 import json
-import uuid
 import logging
 import shutil
+import uuid
+from datetime import datetime, timedelta, timezone
 from pathlib import Path
-from datetime import datetime, timezone, timedelta
-from typing import Optional, List
 
 from .password import hash_password
 
@@ -54,12 +53,12 @@ def has_users() -> bool:
     return bool(_read()["users"])
 
 
-def get_user(username: str) -> Optional[dict]:
+def get_user(username: str) -> dict | None:
     """Get a user by username. Returns None if not found."""
     return _read()["users"].get(username)
 
 
-def get_all_users() -> List[dict]:
+def get_all_users() -> list[dict]:
     """Get all users WITHOUT password_hash (safe for API responses)."""
     users = _read()["users"]
     return [
@@ -72,8 +71,8 @@ def create_user(
     username: str,
     password: str,
     role: str = "user",
-    vaults: Optional[List[str]] = None,
-    display_name: Optional[str] = None,
+    vaults: list[str] | None = None,
+    display_name: str | None = None,
 ) -> dict:
     """Create a new user. Raises ValueError if username already taken."""
     data = _read()