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/routes/api/__init__.py

@@ -0,0 +1,13 @@
+# app/modules/billing/routes/api/__init__.py
+"""
+Billing module API routes.
+
+Provides REST API endpoints for subscription and billing management:
+- Admin API: Subscription tier management, vendor subscriptions, billing history
+- Vendor API: Subscription status, tier comparison, invoices
+"""
+
+from app.modules.billing.routes.api.admin import admin_router
+from app.modules.billing.routes.api.vendor import vendor_router
+
+__all__ = ["admin_router", "vendor_router"]