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/payments/routes/__init__.py

@@ -1,7 +1,11 @@
 # app/modules/payments/routes/__init__.py
-"""Payments module routes."""
+"""
+Payments module routes.
 
-from app.modules.payments.routes.admin import admin_router
-from app.modules.payments.routes.vendor import vendor_router
+Re-exports routers from the api subdirectory for backwards compatibility.
+"""
+
+from app.modules.payments.routes.api.admin import admin_router
+from app.modules.payments.routes.api.vendor import vendor_router
 
 __all__ = ["admin_router", "vendor_router"]