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/customers/locales/de.json

@@ -1 +1,21 @@
-{}
+{
+  "customers": {
+    "title": "Kunden",
+    "customer": "Kunde",
+    "add_customer": "Kunde hinzufügen",
+    "edit_customer": "Kunde bearbeiten",
+    "customer_name": "Kundenname",
+    "customer_email": "Kunden-E-Mail",
+    "customer_phone": "Kundentelefon",
+    "customer_number": "Kundennummer",
+    "first_name": "Vorname",
+    "last_name": "Nachname",
+    "company": "Firma",
+    "total_orders": "Bestellungen gesamt",
+    "total_spent": "Gesamtausgaben",
+    "last_order": "Letzte Bestellung",
+    "registered": "Registriert",
+    "no_customers": "Keine Kunden gefunden",
+    "search_customers": "Kunden suchen..."
+  }
+}