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/__init__.py

@@ -2,27 +2,13 @@
 """
 Billing module route registration.
 
-This module provides functions to register billing routes
-with module-based access control.
+This module provides billing routes with module-based access control.
 
-NOTE: Routers are NOT auto-imported to avoid circular dependencies.
-Import directly from admin.py or vendor.py as needed:
-  from app.modules.billing.routes.admin import admin_router
-  from app.modules.billing.routes.vendor import vendor_router
+Structure:
+- routes/api/ - REST API endpoints
+- routes/pages/ - HTML page rendering (templates)
 """
 
-# Routers are imported on-demand to avoid circular dependencies
-# Do NOT add auto-imports here
+from app.modules.billing.routes.api import admin_router, vendor_router
 
 __all__ = ["admin_router", "vendor_router"]
-
-
-def __getattr__(name: str):
-    """Lazy import routers to avoid circular dependencies."""
-    if name == "admin_router":
-        from app.modules.billing.routes.admin import admin_router
-        return admin_router
-    elif name == "vendor_router":
-        from app.modules.billing.routes.vendor import vendor_router
-        return vendor_router
-    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")

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"]

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}")