refactor: complete module-driven architecture migration

This commit completes the migration to a fully module-driven architecture:

## Models Migration
- Moved all domain models from models/database/ to their respective modules:
  - tenancy: User, Admin, Vendor, Company, Platform, VendorDomain, etc.
  - cms: MediaFile, VendorTheme
  - messaging: Email, VendorEmailSettings, VendorEmailTemplate
  - core: AdminMenuConfig
- models/database/ now only contains Base and TimestampMixin (infrastructure)

## Schemas Migration
- Moved all domain schemas from models/schema/ to their respective modules:
  - tenancy: company, vendor, admin, team, vendor_domain
  - cms: media, image, vendor_theme
  - messaging: email
- models/schema/ now only contains base.py and auth.py (infrastructure)

## Routes Migration
- Moved admin routes from app/api/v1/admin/ to modules:
  - menu_config.py -> core module
  - modules.py -> tenancy module
  - module_config.py -> tenancy module
- app/api/v1/admin/ now only aggregates auto-discovered module routes

## Menu System
- Implemented module-driven menu system with MenuDiscoveryService
- Extended FrontendType enum: PLATFORM, ADMIN, VENDOR, STOREFRONT
- Added MenuItemDefinition and MenuSectionDefinition dataclasses
- Each module now defines its own menu items in definition.py
- MenuService integrates with MenuDiscoveryService for template rendering

## Documentation
- Updated docs/architecture/models-structure.md
- Updated docs/architecture/menu-management.md
- Updated architecture validation rules for new exceptions

## Architecture Validation
- Updated MOD-019 rule to allow base.py in models/schema/
- Created core module exceptions.py and schemas/ directory
- All validation errors resolved (only warnings remain)

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

app/modules/billing/locales/lb.json

@@ -89,7 +89,17 @@
     "payment_method_updated": "Zuelungsmethod aktualiséiert",
     "subscription_cancelled": "Abonnement gekënnegt",
     "error_loading": "Feeler beim Lueden vun de Rechnungsinformatiounen",
-    "error_updating": "Feeler beim Aktualiséieren vum Abonnement"
+    "error_updating": "Feeler beim Aktualiséieren vum Abonnement",
+    "failed_to_load_billing_data": "Failed to load billing data",
+    "failed_to_create_checkout_session": "Failed to create checkout session",
+    "failed_to_open_payment_portal": "Failed to open payment portal",
+    "subscription_cancelled_you_have_access_u": "Subscription cancelled. You have access until the end of your billing period.",
+    "failed_to_cancel_subscription": "Failed to cancel subscription",
+    "subscription_reactivated": "Subscription reactivated!",
+    "failed_to_reactivate_subscription": "Failed to reactivate subscription",
+    "failed_to_purchase_addon": "Failed to purchase add-on",
+    "addon_cancelled_successfully": "Add-on cancelled successfully",
+    "failed_to_cancel_addon": "Failed to cancel add-on"
   },
   "limits": {
     "orders_exceeded": "Monatlech Bestellungslimit erreecht. Upgrade fir méi.",