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/models/__init__.py

@@ -1,10 +1,9 @@
 # app/modules/billing/models/__init__.py
 """
-Billing module models.
+Billing module database models.
 
-Re-exports subscription models from the central models location.
-Models remain in models/database/ for now to avoid breaking existing imports
-across the codebase. This provides a module-local import path.
+This is the canonical location for billing models. Module models are automatically
+discovered and registered with SQLAlchemy's Base.metadata at startup.
 
 Usage:
     from app.modules.billing.models import (
@@ -15,7 +14,7 @@ Usage:
     )
 """
 
-from models.database.subscription import (
+from app.modules.billing.models.subscription import (
     # Enums
     TierCode,
     SubscriptionStatus,