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/templates/vendor/partials/sidebar.html

@@ -184,6 +184,17 @@ Follows same pattern as admin sidebar
                     <span class="ml-4">Settings</span>
                 </a>
             </li>
+            <li class="relative px-6 py-3">
+                <span x-show="currentPage === 'billing'"
+                      class="absolute inset-y-0 left-0 w-1 bg-purple-600 rounded-tr-lg rounded-br-lg"
+                      aria-hidden="true"></span>
+                <a class="inline-flex items-center w-full text-sm font-semibold transition-colors duration-150 hover:text-gray-800 dark:hover:text-gray-200"
+                   :class="currentPage === 'billing' ? 'text-gray-800 dark:text-gray-100' : ''"
+                   :href="`/vendor/${vendorCode}/billing`">
+                    <span x-html="$icon('credit-card', 'w-5 h-5')"></span>
+                    <span class="ml-4">Billing</span>
+                </a>
+            </li>
         </ul>
 
         <!-- Quick Actions -->