feat: complete billing module self-containment

Migrate billing module routes to self-contained structure:

- routes/api/admin.py - Admin API endpoints
- routes/api/vendor.py - Vendor API endpoints
- routes/pages/ - Page routes (placeholder)
- models/subscription.py - Subscription model (moved)
- schemas/subscription.py - Pydantic schemas (moved)
- locales/ - Translations (en, de, fr, lu)

Removed legacy route files:
- app/modules/billing/routes/admin.py
- app/modules/billing/routes/vendor.py

Updated __init__.py files to use new structure.

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

app/modules/billing/definition.py

@@ -12,14 +12,14 @@ from models.database.admin_menu_config import FrontendType
 
 def _get_admin_router():
     """Lazy import of admin router to avoid circular imports."""
-    from app.modules.billing.routes.admin import admin_router
+    from app.modules.billing.routes.api.admin import admin_router
 
     return admin_router
 
 
 def _get_vendor_router():
     """Lazy import of vendor router to avoid circular imports."""
-    from app.modules.billing.routes.vendor import vendor_router
+    from app.modules.billing.routes.api.vendor import vendor_router
 
     return vendor_router