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>