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/main.py

@@ -11,7 +11,7 @@ This module provides:
 from fastapi import APIRouter
 
 from app.api.v1 import admin, shop, vendor
-from app.api.v1.shared import language
+from app.api.v1.shared import language, webhooks
 
 api_router = APIRouter()
 
@@ -42,3 +42,4 @@ api_router.include_router(shop.router, prefix="/v1/shop", tags=["shop"])
 # ============================================================================
 
 api_router.include_router(language.router, prefix="/v1", tags=["language"])
+api_router.include_router(webhooks.router, prefix="/v1", tags=["webhooks"])