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/services/vendor_team_service.py

@@ -20,11 +20,11 @@ from app.core.permissions import get_preset_permissions
 from app.exceptions import (
     CannotRemoveOwnerException,
     InvalidInvitationTokenException,
-    MaxTeamMembersReachedException,
     TeamInvitationAlreadyAcceptedException,
     TeamMemberAlreadyExistsException,
     UserNotFoundException,
 )
+from app.services.subscription_service import TierLimitExceededException
 from middleware.auth import AuthManager
 from models.database.user import User
 from models.database.vendor import Role, Vendor, VendorUser, VendorUserType
@@ -37,7 +37,6 @@ class VendorTeamService:
 
     def __init__(self):
         self.auth_manager = AuthManager()
-        self.max_team_members = 50  # Configure as needed
 
     def invite_team_member(
         self,
@@ -68,21 +67,10 @@ class VendorTeamService:
             Dict with invitation details
         """
         try:
-            # Check team size limit
-            current_team_size = (
-                db.query(VendorUser)
-                .filter(
-                    VendorUser.vendor_id == vendor.id,
-                    VendorUser.is_active == True,
-                )
-                .count()
-            )
+            # Check team size limit from subscription
+            from app.services.subscription_service import subscription_service
 
-            if current_team_size >= self.max_team_members:
-                raise MaxTeamMembersReachedException(
-                    self.max_team_members,
-                    vendor.vendor_code,
-                )
+            subscription_service.check_team_limit(db, vendor.id)
 
             # Check if user already exists
             user = db.query(User).filter(User.email == email).first()
@@ -187,7 +175,7 @@ class VendorTeamService:
                 "existing_user": user.is_active,
             }
 
-        except (TeamMemberAlreadyExistsException, MaxTeamMembersReachedException):
+        except (TeamMemberAlreadyExistsException, TierLimitExceededException):
             raise
         except Exception as e:
             logger.error(f"Error inviting team member: {str(e)}")