refactor: Replace fmt.Printf with structured logging using logrus in api-server subscriber and invoice services

- Add logrus import to invoice.go, subscriber_crud.go, subscriber_esim.go, and subscriber_lifecycle.go
- Replace fmt.Printf with logrus.WithError and WithField for structured error logging
- Update invoice email notification placeholder to use logrus.Info with invoice_id field
- Replace eSIM provisioning error print with logrus.Warn including subscriber_id field
- Add comment explaining derived

apps/api-server/internal/services/subscriber_lifecycle.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/nutcas3/telecom-platform/apps/api-server/internal/models"
+	"github.com/sirupsen/logrus"
 )
 
 // SuspendSubscriber suspends a subscriber and terminates their sessions.
@@ -124,7 +125,7 @@ func (s *SubscriberService) terminateSubscriberSessions(ctx context.Context, ims
 		}
 
 		if err := s.amfClient.TerminateSession(ctx, imsi, "Subscriber terminated"); err != nil {
-			fmt.Printf("Failed to notify AMF for session termination: %v\n", err)
+			logrus.WithError(err).WithField("imsi", imsi).Warn("Failed to notify AMF for session termination")
 		}
 	}