fix: store login crash and dashboard misrouted as storefront

- Seed default RBAC roles per store and assign role_id to StoreUser
  records (was never implemented after RBAC Phase 1 cleanup)
- Handle nullable role in auth_service find_user_store and
  get_user_store_role to prevent NoneType crash on login
- Use platform_clean_path instead of clean_path in FrontendTypeMiddleware
  so /store/X/dashboard is detected as STORE, not STOREFRONT

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/core/services/auth_service.py

@@ -131,7 +131,8 @@ class AuthService:
         )
 
         if store_user:
-            return True, store_user.role.name
+            role_name = store_user.role.name if store_user.role else "staff"
+            return True, role_name
 
         return False, None
 
@@ -213,7 +214,8 @@ class AuthService:
                 (vm for vm in user.store_memberships if vm.is_active), None
             )
             if active_membership:
-                return active_membership.store, active_membership.role.name
+                role_name = active_membership.role.name if active_membership.role else "staff"
+                return active_membership.store, role_name
 
         return None, None