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>

models/database/__init__.py

@@ -51,9 +51,16 @@ from .order_item_exception import OrderItemException
 from .product import Product
 from .product_translation import ProductTranslation
 from .subscription import (
+    AddOnCategory,
+    AddOnProduct,
+    BillingHistory,
+    BillingPeriod,
+    StripeWebhookEvent,
     SubscriptionStatus,
+    SubscriptionTier,
     TierCode,
     TIER_LIMITS,
+    VendorAddOn,
     VendorSubscription,
 )
 from .test_run import TestCollection, TestResult, TestRun
@@ -121,11 +128,18 @@ __all__ = [
     "LetzshopFulfillmentQueue",
     "LetzshopSyncLog",
     "LetzshopHistoricalImportJob",
-    # Subscription
+    # Subscription & Billing
     "VendorSubscription",
     "SubscriptionStatus",
+    "SubscriptionTier",
     "TierCode",
     "TIER_LIMITS",
+    "AddOnProduct",
+    "AddOnCategory",
+    "BillingPeriod",
+    "VendorAddOn",
+    "BillingHistory",
+    "StripeWebhookEvent",
     # Messaging
     "Conversation",
     "ConversationParticipant",