feat(sms): implement SMS messaging and metered billing

Implement a complete SMS messaging system including:
- Integration with SMS Manager.cz API for sending messages.
- Metered billing via Stripe using monthly aggregate invoice items.
- Backend services for managing SMS settings, usage logging, and monthly reporting.
- Database migrations for tenant settings, usage logs, and monthly reports.
- Frontend dashboard components for SMS configuration, usage tracking, and history.
- Support for customer phone numbers in the booking flow.

Includes new migrations, backend services, and frontend UI components.

apps/backend/internal/billing/service.go

@@ -780,6 +780,7 @@ func entitlementsForPlan(planCode string) domain.PlanEntitlements {
 			WidgetEmbedding:   true,
 			UmamiTracking:     false,
 			APIAccess:         false,
+			SMSAvailable:      false,
 		}
 	case "business":
 		// Business: Unlimited everything, API access, dedicated manager
@@ -793,6 +794,7 @@ func entitlementsForPlan(planCode string) domain.PlanEntitlements {
 			UmamiTracking:     true,
 			APIAccess:         true,
 			DedicatedManager:  true,
+			SMSAvailable:      true,
 		}
 	default:
 		// Pro: 3 locations, 10 staff, unlimited bookings, email reminders, analytics
@@ -805,6 +807,7 @@ func entitlementsForPlan(planCode string) domain.PlanEntitlements {
 			WidgetEmbedding:   true,
 			UmamiTracking:     true,
 			APIAccess:         false,
+			SMSAvailable:      true,
 		}
 	}
 }