diplomail (Stage D): language detection + lazy translation cache

Replaces the LangUndetermined placeholder with whatlanggo-backed
body detection on every send path, then adds a translation cache
keyed on (message_id, target_lang) populated lazily on the
per-message read endpoint. The noop translator that ships with
Stage D returns engine="noop", which the service treats as
"translation unavailable" — wiring a real backend (LibreTranslate
HTTP client is the documented next step) is a one-file swap.

GetMessage and ListInbox now accept a targetLang argument; the HTTP
layer resolves the caller's accounts.preferred_language and
forwards it. Inbox uses the cache only (never calls the
translator) so bulk reads stay fast under future SaaS backends.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/internal/server/handlers_user_mail.go

@@ -48,6 +48,25 @@ func NewUserMailHandlers(svc *diplomail.Service, lobbySvc *lobby.Service, users
 	}
 }
 
+// preferredLanguage looks up the caller's `accounts.preferred_language`
+// so the per-message read can attach the cached translation when
+// available. Failures are logged at debug level and the function
+// returns an empty string — translation is best-effort and the
+// caller still receives the original body.
+func (h *UserMailHandlers) preferredLanguage(ctx context.Context, userID uuid.UUID) string {
+	if h.users == nil {
+		return ""
+	}
+	account, err := h.users.GetAccount(ctx, userID)
+	if err != nil {
+		h.logger.Debug("resolve preferred_language failed",
+			zap.String("user_id", userID.String()),
+			zap.Error(err))
+		return ""
+	}
+	return account.PreferredLanguage
+}
+
 // SendPersonal handles POST /api/v1/user/games/{game_id}/mail/messages.
 func (h *UserMailHandlers) SendPersonal() gin.HandlerFunc {
 	if h.svc == nil {
@@ -109,7 +128,8 @@ func (h *UserMailHandlers) Get() gin.HandlerFunc {
 			return
 		}
 		ctx := c.Request.Context()
-		entry, err := h.svc.GetMessage(ctx, userID, messageID)
+		targetLang := h.preferredLanguage(ctx, userID)
+		entry, err := h.svc.GetMessage(ctx, userID, messageID, targetLang)
 		if err != nil {
 			respondDiplomailError(c, h.logger, "user mail get", ctx, err)
 			return
@@ -134,7 +154,8 @@ func (h *UserMailHandlers) Inbox() gin.HandlerFunc {
 			return
 		}
 		ctx := c.Request.Context()
-		items, err := h.svc.ListInbox(ctx, gameID, userID)
+		targetLang := h.preferredLanguage(ctx, userID)
+		items, err := h.svc.ListInbox(ctx, gameID, userID, targetLang)
 		if err != nil {
 			respondDiplomailError(c, h.logger, "user mail inbox", ctx, err)
 			return
@@ -491,6 +512,10 @@ func mailBroadcastReceiptToWire(m diplomail.Message, recipients []diplomail.Reci
 // userMailMessageDetailWire mirrors the unified response shape for
 // inbox listings and per-message reads. Sender identifiers are
 // optional: system messages carry neither user id nor username.
+// Translation fields are populated when a cached rendering exists
+// for the caller's `preferred_language`; the UI renders
+// `body_translated` and surfaces the original through a
+// "show original" toggle.
 type userMailMessageDetailWire struct {
 	MessageID         string  `json:"message_id"`
 	GameID            string  `json:"game_id"`
@@ -509,6 +534,10 @@ type userMailMessageDetailWire struct {
 	RecipientRaceName *string `json:"recipient_race_name,omitempty"`
 	ReadAt            *string `json:"read_at,omitempty"`
 	DeletedAt         *string `json:"deleted_at,omitempty"`
+	TranslatedSubject *string `json:"translated_subject,omitempty"`
+	TranslatedBody    *string `json:"translated_body,omitempty"`
+	TranslationLang   *string `json:"translation_lang,omitempty"`
+	Translator        *string `json:"translator,omitempty"`
 }
 
 // userMailSentSummaryWire mirrors the response shape for the
@@ -580,6 +609,17 @@ func mailMessageDetailToWire(entry diplomail.InboxEntry, justCreated bool) userM
 		s := entry.Recipient.DeletedAt.UTC().Format(timestampLayout)
 		out.DeletedAt = &s
 	}
+	if entry.Translation != nil {
+		tr := entry.Translation
+		subj := tr.TranslatedSubject
+		body := tr.TranslatedBody
+		lang := tr.TargetLang
+		engine := tr.Translator
+		out.TranslatedSubject = &subj
+		out.TranslatedBody = &body
+		out.TranslationLang = &lang
+		out.Translator = &engine
+	}
 	_ = justCreated
 	return out
 }