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/api/v1/vendor/__init__.py

@@ -15,13 +15,14 @@ For multi-tenant apps, module enablement is checked at request time
 based on platform context (not at route registration time).
 
 Self-contained modules (auto-discovered from app/modules/{module}/routes/api/vendor.py):
-- billing: Subscription tiers, vendor billing, invoices
+- billing: Subscription tiers, vendor billing, checkout, add-ons, features
 - inventory: Stock management, inventory tracking
-- orders: Order management, fulfillment, exceptions
+- orders: Order management, fulfillment, exceptions, invoices
 - marketplace: Letzshop integration, product sync
 - catalog: Vendor product catalog management
 - cms: Content pages management
 - customers: Customer management
+- payments: Payment configuration, Stripe connect, transactions
 """
 
 from fastapi import APIRouter
@@ -30,18 +31,14 @@ from fastapi import APIRouter
 from . import (
     analytics,
     auth,
-    billing,
     dashboard,
     email_settings,
     email_templates,
-    features,
     info,
-    invoices,
     media,
     messages,
     notifications,
     onboarding,
-    payments,
     profile,
     settings,
     team,
@@ -71,17 +68,14 @@ router.include_router(email_templates.router, tags=["vendor-email-templates"])
 router.include_router(email_settings.router, tags=["vendor-email-settings"])
 router.include_router(onboarding.router, tags=["vendor-onboarding"])
 
-# Business operations (with prefixes: /invoices/*, /team/*)
-router.include_router(invoices.router, tags=["vendor-invoices"])
+# Business operations (with prefixes: /team/*)
 router.include_router(team.router, tags=["vendor-team"])
 
-# Services (with prefixes: /payments/*, /media/*, etc.)
-router.include_router(payments.router, tags=["vendor-payments"])
+# Services (with prefixes: /media/*, etc.)
 router.include_router(media.router, tags=["vendor-media"])
 router.include_router(notifications.router, tags=["vendor-notifications"])
 router.include_router(messages.router, tags=["vendor-messages"])
 router.include_router(analytics.router, tags=["vendor-analytics"])
-router.include_router(features.router, tags=["vendor-features"])
 router.include_router(usage.router, tags=["vendor-usage"])
 
 
@@ -89,7 +83,7 @@ router.include_router(usage.router, tags=["vendor-usage"])
 # Auto-discovered Module Routes
 # ============================================================================
 # Routes from self-contained modules are auto-discovered and registered.
-# Modules include: billing, inventory, orders, marketplace, cms, customers
+# Modules include: billing, inventory, orders, marketplace, cms, customers, payments
 # Routes are sorted by priority, so catch-all routes (CMS) come last.
 
 from app.modules.routes import get_vendor_api_routes