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/enums.py

@@ -0,0 +1,46 @@
+# app/modules/enums.py
+"""
+Module system enums.
+
+This file contains enums used by the module system that need to be
+importable without triggering database model imports.
+
+The FrontendType enum is defined here to break a circular import:
+- app.modules.base imports FrontendType
+- Previously FrontendType was in models.database.admin_menu_config
+- Importing from models.database triggers model discovery
+- Model discovery imports module definitions
+- Module definitions import from app.modules.base → CIRCULAR
+
+By defining FrontendType here, we break this cycle.
+"""
+
+import enum
+
+
+class FrontendType(str, enum.Enum):
+    """Frontend types that can have menu configuration."""
+
+    ADMIN = "admin"    # Admin panel (super admins, platform admins)
+    VENDOR = "vendor"  # Vendor dashboard
+
+
+# Menu items that cannot be hidden - always visible regardless of config
+# Organized by frontend type
+MANDATORY_MENU_ITEMS = {
+    FrontendType.ADMIN: frozenset({
+        "dashboard",  # Default landing page after login
+        "companies",
+        "vendors",
+        "admin-users",
+        "settings",
+        "my-menu",  # Super admin menu config - must always be accessible
+    }),
+    FrontendType.VENDOR: frozenset({
+        "dashboard",  # Default landing page after login
+        "settings",
+    }),
+}
+
+
+__all__ = ["FrontendType", "MANDATORY_MENU_ITEMS"]