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/schemas/__init__.py

@@ -1,9 +1,8 @@
 # app/modules/billing/schemas/__init__.py
 """
-Billing module Pydantic schemas.
+Billing module Pydantic schemas for API request/response validation.
 
-Re-exports subscription schemas from the central schemas location.
-Provides a module-local import path while maintaining backwards compatibility.
+This is the canonical location for billing schemas.
 
 Usage:
     from app.modules.billing.schemas import (
@@ -13,7 +12,7 @@ Usage:
     )
 """
 
-from models.schema.subscription import (
+from app.modules.billing.schemas.subscription import (
     # Tier schemas
     TierFeatures,
     TierLimits,