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/routes/vendor_pages.py

@@ -454,6 +454,28 @@ async def vendor_settings_page(
     )
 
 
+@router.get(
+    "/{vendor_code}/billing", response_class=HTMLResponse, include_in_schema=False
+)
+async def vendor_billing_page(
+    request: Request,
+    vendor_code: str = Path(..., description="Vendor code"),
+    current_user: User = Depends(get_current_vendor_from_cookie_or_header),
+):
+    """
+    Render billing and subscription management page.
+    JavaScript loads subscription status, tiers, and invoices via API.
+    """
+    return templates.TemplateResponse(
+        "vendor/billing.html",
+        {
+            "request": request,
+            "user": current_user,
+            "vendor_code": vendor_code,
+        },
+    )
+
+
 # ============================================================================
 # DYNAMIC CONTENT PAGES (CMS)
 # ============================================================================