feat: mail service

authsession/internal/domain/challenge/model.go

@@ -5,6 +5,7 @@ package challenge
 import (
 	"errors"
 	"fmt"
+	"strings"
 	"time"
 
 	"galaxy/authsession/internal/domain/common"
@@ -239,6 +240,10 @@ type Challenge struct {
 	// CodeHash stores only the hashed confirmation code.
 	CodeHash []byte
 
+	// PreferredLanguage stores the canonical create-only preferred-language
+	// candidate derived when the challenge was created.
+	PreferredLanguage string
+
 	// Status reports the coarse challenge lifecycle state.
 	Status Status
 
@@ -279,6 +284,12 @@ func (c Challenge) Validate() error {
 	if len(c.CodeHash) == 0 {
 		return errors.New("challenge code hash must not be empty")
 	}
+	if strings.TrimSpace(c.PreferredLanguage) == "" {
+		return errors.New("challenge preferred language must not be empty")
+	}
+	if strings.TrimSpace(c.PreferredLanguage) != c.PreferredLanguage {
+		return errors.New("challenge preferred language must not contain surrounding whitespace")
+	}
 	if !c.Status.IsKnown() {
 		return fmt.Errorf("challenge status %q is unsupported", c.Status)
 	}

authsession/internal/domain/challenge/model_test.go

@@ -404,13 +404,14 @@ func validChallenge(t *testing.T) Challenge {
 	t.Helper()
 
 	return Challenge{
-		ID:            common.ChallengeID("challenge-123"),
-		Email:         common.Email("pilot@example.com"),
-		CodeHash:      []byte("hash-123"),
-		Status:        StatusPendingSend,
-		DeliveryState: DeliveryPending,
-		CreatedAt:     time.Unix(1_775_121_600, 0).UTC(),
-		ExpiresAt:     time.Unix(1_775_121_900, 0).UTC(),
+		ID:                common.ChallengeID("challenge-123"),
+		Email:             common.Email("pilot@example.com"),
+		CodeHash:          []byte("hash-123"),
+		PreferredLanguage: "en",
+		Status:            StatusPendingSend,
+		DeliveryState:     DeliveryPending,
+		CreatedAt:         time.Unix(1_775_121_600, 0).UTC(),
+		ExpiresAt:         time.Unix(1_775_121_900, 0).UTC(),
 		Attempts: AttemptCounters{
 			Send:    0,
 			Confirm: 0,