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_test.go

@@ -52,7 +52,7 @@ func TestCreateCheckoutRequiresPaddleConfig(t *testing.T) {
 	response, err := service.CreateCheckoutSession(context.Background(), domain.Principal{
 		Subject: "demo-owner",
 		Email:   "owner@bookra.dev",
-	}, "pro", "czk")
+	}, "pro", "czk", "monthly")
 	if err != ErrPaddleNotConfigured {
 		t.Fatalf("expected ErrPaddleNotConfigured, got response=%v err=%v", response, err)
 	}
@@ -64,7 +64,7 @@ func TestCreateCheckoutReturnsLaunchPayload(t *testing.T) {
 	response, err := service.CreateCheckoutSession(context.Background(), domain.Principal{
 		Subject: "demo-owner",
 		Email:   "owner@bookra.dev",
-	}, "pro", "czk")
+	}, "pro", "czk", "monthly")
 	if err != nil {
 		t.Fatalf("create checkout: %v", err)
 	}