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

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/admin/products.py

@@ -101,6 +101,8 @@ class CopyToVendorResponse(BaseModel):
     copied: int
     skipped: int
     failed: int
+    auto_matched: int = 0
+    limit_reached: bool = False
     details: list[dict] | None = None