Add email logging and SMTP health check

- Add structured logging for email sending (success/failure)
- Add HealthCheck method to EmailService interface
- Extend /health endpoint to include SMTP status
- Update all email service mocks to implement HealthCheck
- SMTP status shows: ok, error, or disabled

internal/domain/services/email_service.go

@@ -9,4 +9,5 @@ type EmailMessage struct {
 
 type EmailService interface {
 	Send(message EmailMessage) error
+	HealthCheck() error
 }