feat(registration): add email domain whitelist policy

backend/internal/service/registration_email_policy_test.go

@@ -0,0 +1,31 @@
+//go:build unit
+
+package service
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestNormalizeRegistrationEmailSuffixWhitelist(t *testing.T) {
+	got, err := NormalizeRegistrationEmailSuffixWhitelist([]string{"example.com", "@EXAMPLE.COM", " @foo.bar "})
+	require.NoError(t, err)
+	require.Equal(t, []string{"@example.com", "@foo.bar"}, got)
+}
+
+func TestNormalizeRegistrationEmailSuffixWhitelist_Invalid(t *testing.T) {
+	_, err := NormalizeRegistrationEmailSuffixWhitelist([]string{"@invalid_domain"})
+	require.Error(t, err)
+}
+
+func TestParseRegistrationEmailSuffixWhitelist(t *testing.T) {
+	got := ParseRegistrationEmailSuffixWhitelist(`["example.com","@foo.bar","@invalid_domain"]`)
+	require.Equal(t, []string{"@example.com", "@foo.bar"}, got)
+}
+
+func TestIsRegistrationEmailSuffixAllowed(t *testing.T) {
+	require.True(t, IsRegistrationEmailSuffixAllowed("user@example.com", []string{"@example.com"}))
+	require.False(t, IsRegistrationEmailSuffixAllowed("user@sub.example.com", []string{"@example.com"}))
+	require.True(t, IsRegistrationEmailSuffixAllowed("user@any.com", []string{}))
+}