feat: add subscription and billing system with Stripe integration

- Add database models for subscription tiers, vendor subscriptions,
  add-ons, billing history, and webhook events
- Implement BillingService for subscription operations
- Implement StripeService for Stripe API operations
- Implement StripeWebhookHandler for webhook event processing
- Add vendor billing API endpoints for subscription management
- Create vendor billing page with Alpine.js frontend
- Add limit enforcement for products and team members
- Add billing exceptions for proper error handling
- Create comprehensive unit tests (40 tests passing)
- Add subscription billing documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/api/v1/vendor/__init__.py

@@ -16,6 +16,7 @@ from fastapi import APIRouter
 from . import (
     analytics,
     auth,
+    billing,
     content_pages,
     customers,
     dashboard,
@@ -73,6 +74,7 @@ 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(billing.router, tags=["vendor-billing"])
 
 # Content pages management
 router.include_router(