style: run gofmt -s across all Go files

Fixes Go Report Card gofmt score from 52% to 100%.
Pure formatting changes — no logic modifications.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/service/notification.go

@@ -11,7 +11,7 @@ import (
 
 // NotificationService provides business logic for managing notifications.
 type NotificationService struct {
-	notifRepo       repository.NotificationRepository
+	notifRepo        repository.NotificationRepository
 	notifierRegistry map[string]Notifier
 }