Deep-scrub secrets from public settings

/api/auth/settings is auth-exempt (the frontend + the pre-login page read it for
keybinds/TTS prefs), so non-admin and unauthenticated callers get a scrubbed
copy. The previous scrub only blanked TOP-LEVEL string values whose key matched a
short suffix list — so a secret nested under a non-secret parent key, or stored
under a key outside the list, would leak. A real exposure when the app is
reachable over a Cloudflare tunnel / reverse proxy.

- src/settings_scrub.py: NEW stdlib-only module with the scrub helpers (deep/
  recursive; broadened secret-key patterns). Kept separate from auth_routes so it
  imports + unit-tests WITHOUT pulling the FastAPI / auth / database chain
  (addresses review: the test no longer fails at collection on the DB import).
- routes/auth_routes.py: import scrub_settings from the module.
- tests/test_settings_scrub.py: import the tiny module directly.

Ran: pytest tests/test_settings_scrub.py (8 passed); verified the test pulls no
db/auth modules into sys.modules; py_compile routes/auth_routes.py.

Co-authored-by: Kanaru92 <107661007+Kanaru92@users.noreply.github.com>

routes/auth_routes.py

@@ -8,6 +8,7 @@ import os
 
 from core.auth import AuthManager
 from src.rate_limiter import RateLimiter
+from src.settings_scrub import scrub_settings
 from src.settings import (
     load_settings as _load_settings,
     save_settings as _save_settings,
@@ -371,29 +372,6 @@ def setup_auth_routes(auth_manager: AuthManager) -> APIRouter:
 
     # ---- App settings (admin-managed) ----
 
-    _SECRET_KEY_PATTERNS = ("_api_key", "_password", "_secret", "_token", "_key")
-
-    def _is_secret_key(name: str) -> bool:
-        n = (name or "").lower()
-        if n in ("google_pse_cx",):  # public identifier, not a secret
-            return False
-        return any(n.endswith(p) or n == p.lstrip("_") for p in _SECRET_KEY_PATTERNS)
-
-    def _scrub_settings(settings: dict) -> dict:
-        """Return a copy of settings with secret-shaped values masked.
-
-        Frontend reads /settings without auth for things like keybinds + TTS
-        prefs. Secrets (search-provider keys, IMAP/SMTP passwords) must NOT
-        be exposed to non-admin callers.
-        """
-        scrubbed = {}
-        for k, v in (settings or {}).items():
-            if _is_secret_key(k) and isinstance(v, str) and v:
-                scrubbed[k] = ""  # presence preserved, value blanked
-            else:
-                scrubbed[k] = v
-        return scrubbed
-
     @router.get("/settings")
     async def get_settings(request: Request):
         """Returns app settings. Admins get the full set; non-admins get
@@ -403,7 +381,7 @@ def setup_auth_routes(auth_manager: AuthManager) -> APIRouter:
         settings = _load_settings()
         if user and auth_manager.is_admin(user):
             return settings
-        return _scrub_settings(settings)
+        return scrub_settings(settings)
 
     @router.post("/settings")
     async def set_settings(request: Request):