Log redacted email provider errors and diagnose sender configuration (#57)

backend/notifications/email/driver.go

@@ -33,6 +33,10 @@ type Outcome struct {
 	State      model.State
 	Code       string
 	RetryAfter time.Duration
+	// Provider diagnostics are sanitized operational data, not public failures.
+	HTTPStatus      int
+	ProviderError   string
+	ProviderMessage string
 }
 type Sender interface {
 	Send(context.Context, string, model.Email) Outcome