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/definition.py

@@ -21,14 +21,14 @@ from models.database.admin_menu_config import FrontendType
 
 def _get_admin_router():
     """Lazy import of admin router to avoid circular imports."""
-    from app.modules.payments.routes.admin import admin_router
+    from app.modules.payments.routes.api.admin import admin_router
 
     return admin_router
 
 
 def _get_vendor_router():
     """Lazy import of vendor router to avoid circular imports."""
-    from app.modules.payments.routes.vendor import vendor_router
+    from app.modules.payments.routes.api.vendor import vendor_router
 
     return vendor_router
 
@@ -61,6 +61,7 @@ payments_module = ModuleDefinition(
     },
     is_core=False,
     is_internal=False,
+    is_self_contained=True,  # Enable auto-discovery from routes/api/
 )