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/billing/routes/api/vendor.py

@@ -213,3 +213,17 @@ def get_invoices(
 # NOTE: Additional endpoints (checkout, portal, cancel, addons, etc.)
 # are still handled by app/api/v1/vendor/billing.py for now.
 # They can be migrated here as part of a larger refactoring effort.
+
+
+# ============================================================================
+# Aggregate Sub-Routers
+# ============================================================================
+# Include all billing-related vendor sub-routers
+
+from app.modules.billing.routes.api.vendor_features import vendor_features_router
+from app.modules.billing.routes.api.vendor_checkout import vendor_checkout_router
+from app.modules.billing.routes.api.vendor_addons import vendor_addons_router
+
+vendor_router.include_router(vendor_features_router, tags=["vendor-features"])
+vendor_router.include_router(vendor_checkout_router, tags=["vendor-billing"])
+vendor_router.include_router(vendor_addons_router, tags=["vendor-billing-addons"])