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/pages/__init__.py

@@ -0,0 +1,18 @@
+# app/modules/billing/routes/pages/__init__.py
+"""
+Billing module page routes (HTML rendering).
+
+Provides Jinja2 template rendering for billing management:
+- Admin pages: Subscription tiers, subscriptions list, billing history
+- Vendor pages: Billing dashboard, invoices
+
+Note: These routes are placeholders. The actual page rendering
+is currently handled by routes in app/api/v1/ and can be migrated here.
+"""
+
+__all__ = []
+
+
+def __getattr__(name: str):
+    """Lazy import routers to avoid circular dependencies."""
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")