fix: resolve circular import in module system

The circular import occurred because:
1. app.modules.base imported FrontendType from models.database.admin_menu_config
2. This triggered models/database/__init__.py which runs model discovery
3. Model discovery imported module definitions
4. Module definitions imported from app.modules.base (still initializing)

Solution: Move FrontendType and MANDATORY_MENU_ITEMS to a new
app/modules/enums.py file. The models file re-exports them for
backward compatibility with existing imports.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/base.py

@@ -44,7 +44,7 @@ if TYPE_CHECKING:
     from fastapi import APIRouter
     from pydantic import BaseModel
 
-from models.database.admin_menu_config import FrontendType
+from app.modules.enums import FrontendType
 
 
 @dataclass