fix(critical): permission cache returns None causing 500 on every authenticated API call

- get_cached_permissions() returned None when _get_current_permission_version failed
- deps.py get_current_user() crashed with AttributeError: NoneType.get()
- Fix: fall through to DB resolution instead of returning None
- Fix: add None guard in deps.py as safety net

app/core/permissions.py

@@ -340,7 +340,7 @@ async def get_cached_permissions(
                         exc_info=True,
                     )
                     await redis.delete(cache_key)
-                    return None  # Fall through to re-resolution from DB
+                    # Fall through to re-resolution from DB (don't return None)
 
                 if cached_version == current_version:
                     return data