refactor: migrate vendor billing, invoices, payments to module auto-discovery

Billing module:
- Create vendor_checkout.py (checkout, portal, cancel, reactivate, change-tier)
- Create vendor_addons.py (addon management routes)
- Update vendor.py to aggregate new routers

Orders module:
- Create vendor_invoices.py (invoice settings, CRUD, PDF generation)
- Update vendor.py to aggregate invoices router

Payments module:
- Restructure routes from routes/ to routes/api/
- Add require_module_access dependency
- Set is_self_contained=True for auto-discovery

Remove legacy files:
- app/api/v1/vendor/billing.py
- app/api/v1/vendor/invoices.py
- app/api/v1/vendor/payments.py

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/orders/routes/api/vendor.py

@@ -282,9 +282,11 @@ def ship_order_item(
 # Aggregate routers
 # ============================================================================
 
-# Import exceptions router
+# Import sub-routers
 from app.modules.orders.routes.api.vendor_exceptions import vendor_exceptions_router
+from app.modules.orders.routes.api.vendor_invoices import vendor_invoices_router
 
-# Include both routers into the aggregate vendor_router
+# Include all sub-routers into the aggregate vendor_router
 vendor_router.include_router(_orders_router, tags=["vendor-orders"])
 vendor_router.include_router(vendor_exceptions_router, tags=["vendor-order-exceptions"])
+vendor_router.include_router(vendor_invoices_router, tags=["vendor-invoices"])