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/exceptions/__init__.py

@@ -46,6 +46,18 @@ from .base import (
     WizamartException,
 )
 
+# Billing exceptions
+from .billing import (
+    InvalidWebhookSignatureException,
+    NoActiveSubscriptionException,
+    PaymentSystemNotConfiguredException,
+    StripePriceNotConfiguredException,
+    SubscriptionAlreadyCancelledException,
+    SubscriptionNotCancelledException,
+    TierNotFoundException,
+    WebhookMissingSignatureException,
+)
+
 # Cart exceptions
 from .cart import (
     CartItemNotFoundException,
@@ -416,4 +428,13 @@ __all__ = [
     "InvalidConversationTypeException",
     "InvalidRecipientTypeException",
     "AttachmentNotFoundException",
+    # Billing exceptions
+    "PaymentSystemNotConfiguredException",
+    "TierNotFoundException",
+    "StripePriceNotConfiguredException",
+    "NoActiveSubscriptionException",
+    "SubscriptionNotCancelledException",
+    "SubscriptionAlreadyCancelledException",
+    "InvalidWebhookSignatureException",
+    "WebhookMissingSignatureException",
 ]