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>

mkdocs.yml

@@ -204,6 +204,7 @@ nav:
       - Implementation Guide: features/cms-implementation-guide.md
     - Platform Homepage: features/platform-homepage.md
     - Vendor Landing Pages: features/vendor-landing-pages.md
+    - Subscription & Billing: features/subscription-billing.md
 
   # ============================================
   # USER GUIDES