feat: extract billing module with routes (Phase 3)

Create app/modules/billing/ directory structure with:
- definition.py: Module definition with features and menu items
- routes/admin.py: Admin billing routes with module access control
- routes/vendor.py: Vendor billing routes with module access control

Key changes:
- Billing module uses require_module_access("billing") dependency
- Admin router now includes billing module router instead of legacy
- Module registry imports billing_module from extracted location
- Routes have identical functionality but are now module-gated

Module structure pattern for future extractions:
  app/modules/{module}/
  ├── __init__.py
  ├── definition.py (ModuleDefinition + router getters)
  └── routes/
      ├── __init__.py
      ├── admin.py (require_module_access dependency)
      └── vendor.py (require_module_access dependency)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/admin/__init__.py

@@ -19,6 +19,20 @@ IMPORTANT:
 - This router is for JSON API endpoints only
 - HTML page routes are mounted separately in main.py at /vendor/*
 - Do NOT include pages.router here - it causes route conflicts
+
+MODULE SYSTEM:
+Routes can be module-gated using require_module_access() dependency.
+For multi-tenant apps, module enablement is checked at request time
+based on platform context (not at route registration time).
+
+Extracted modules (app/modules/{module}/routes/):
+- billing: Subscription tiers, vendor billing, invoices
+
+Future extractions will follow the same pattern:
+1. Create app/modules/{module}/ directory
+2. Move routes to app/modules/{module}/routes/admin.py
+3. Add require_module_access("{module}") to router
+4. Update this file to import from module instead
 """
 
 from fastapi import APIRouter
@@ -42,6 +56,7 @@ from . import (
     logs,
     marketplace,
     media,
+    menu_config,
     messages,
     monitoring,
     notifications,
@@ -51,7 +66,7 @@ from . import (
     platforms,
     products,
     settings,
-    subscriptions,
+    subscriptions,  # Legacy - will be replaced by billing module router
     tests,
     users,
     vendor_domains,
@@ -60,6 +75,9 @@ from . import (
     vendors,
 )
 
+# Import extracted module routers
+from app.modules.billing.routes import admin_router as billing_admin_router
+
 # Create admin router
 router = APIRouter()
 
@@ -96,6 +114,9 @@ router.include_router(
 # Include platforms management endpoints (multi-platform CMS)
 router.include_router(platforms.router, tags=["admin-platforms"])
 
+# Include menu configuration endpoints (super admin only)
+router.include_router(menu_config.router, tags=["admin-menu-config"])
+
 
 # ============================================================================
 # User Management
@@ -192,11 +213,15 @@ router.include_router(
 
 
 # ============================================================================
-# Billing & Subscriptions
+# Billing & Subscriptions (Module-gated)
 # ============================================================================
 
-# Include subscription management endpoints
-router.include_router(subscriptions.router, tags=["admin-subscriptions"])
+# Include billing module router (with module access control)
+# This router checks if the 'billing' module is enabled for the platform
+router.include_router(billing_admin_router, tags=["admin-billing"])
+
+# Legacy subscriptions router (to be removed once billing module is fully tested)
+# router.include_router(subscriptions.router, tags=["admin-subscriptions"])
 
 # Include feature management endpoints
 router.include_router(features.router, tags=["admin-features"])