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/billing.py

@@ -0,0 +1,95 @@
+# app/exceptions/billing.py
+"""
+Billing and subscription related exceptions.
+
+This module provides exceptions for:
+- Payment system configuration issues
+- Subscription management errors
+- Tier-related errors
+"""
+
+from typing import Any
+
+from .base import BusinessLogicException, ResourceNotFoundException, ServiceUnavailableException
+
+
+class PaymentSystemNotConfiguredException(ServiceUnavailableException):
+    """Raised when the payment system (Stripe) is not configured."""
+
+    def __init__(self):
+        super().__init__(message="Payment system not configured")
+
+
+class TierNotFoundException(ResourceNotFoundException):
+    """Raised when a subscription tier is not found."""
+
+    def __init__(self, tier_code: str):
+        super().__init__(
+            resource_type="SubscriptionTier",
+            identifier=tier_code,
+            message=f"Subscription tier '{tier_code}' not found",
+            error_code="TIER_NOT_FOUND",
+        )
+        self.tier_code = tier_code
+
+
+class StripePriceNotConfiguredException(BusinessLogicException):
+    """Raised when Stripe price is not configured for a tier."""
+
+    def __init__(self, tier_code: str):
+        super().__init__(
+            message=f"Stripe price not configured for tier '{tier_code}'",
+            error_code="STRIPE_PRICE_NOT_CONFIGURED",
+            details={"tier_code": tier_code},
+        )
+        self.tier_code = tier_code
+
+
+class NoActiveSubscriptionException(BusinessLogicException):
+    """Raised when no active subscription exists for an operation that requires one."""
+
+    def __init__(self, message: str = "No active subscription found"):
+        super().__init__(
+            message=message,
+            error_code="NO_ACTIVE_SUBSCRIPTION",
+        )
+
+
+class SubscriptionNotCancelledException(BusinessLogicException):
+    """Raised when trying to reactivate a subscription that is not cancelled."""
+
+    def __init__(self):
+        super().__init__(
+            message="Subscription is not cancelled",
+            error_code="SUBSCRIPTION_NOT_CANCELLED",
+        )
+
+
+class SubscriptionAlreadyCancelledException(BusinessLogicException):
+    """Raised when trying to cancel an already cancelled subscription."""
+
+    def __init__(self):
+        super().__init__(
+            message="Subscription is already cancelled",
+            error_code="SUBSCRIPTION_ALREADY_CANCELLED",
+        )
+
+
+class InvalidWebhookSignatureException(BusinessLogicException):
+    """Raised when Stripe webhook signature verification fails."""
+
+    def __init__(self, message: str = "Invalid webhook signature"):
+        super().__init__(
+            message=message,
+            error_code="INVALID_WEBHOOK_SIGNATURE",
+        )
+
+
+class WebhookMissingSignatureException(BusinessLogicException):
+    """Raised when Stripe webhook is missing the signature header."""
+
+    def __init__(self):
+        super().__init__(
+            message="Missing Stripe-Signature header",
+            error_code="WEBHOOK_MISSING_SIGNATURE",
+        )