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/vendor.py

@@ -166,6 +166,21 @@ class Vendor(Base, TimestampMixin):
         cascade="all, delete-orphan",
     )
 
+    # Add-ons purchased by vendor (one-to-many)
+    addons = relationship(
+        "VendorAddOn",
+        back_populates="vendor",
+        cascade="all, delete-orphan",
+    )
+
+    # Billing/invoice history (one-to-many)
+    billing_history = relationship(
+        "BillingHistory",
+        back_populates="vendor",
+        cascade="all, delete-orphan",
+        order_by="BillingHistory.invoice_date.desc()",
+    )
+
     domains = relationship(
         "VendorDomain",
         back_populates="vendor",