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/diplomail/service.go

@@ -68,7 +68,7 @@ func (s *Service) SendPersonal(ctx context.Context, in SendPersonalInput) (Messa
 		SenderIP:       in.SenderIP,
 		Subject:        subject,
 		Body:           body,
-		BodyLang:       LangUndetermined,
+		BodyLang:       s.deps.Detector.Detect(body),
 		BroadcastScope: BroadcastScopeSingle,
 	}
 	raceName := recipient.RaceName
@@ -104,7 +104,15 @@ func (s *Service) SendPersonal(ctx context.Context, in SendPersonalInput) (Messa
 // the caller is no longer an active member of the game and the
 // message is personal-kind: post-kick visibility is restricted to
 // admin/system mail (item 8 of the spec).
-func (s *Service) GetMessage(ctx context.Context, userID, messageID uuid.UUID) (InboxEntry, error) {
+//
+// When `targetLang` is non-empty and differs from the message's
+// `body_lang`, the function consults the translation cache; on a
+// miss it asks the configured Translator to produce a rendering and
+// persists the result. The noop translator returns the input
+// unchanged with `engine == "noop"`, which is treated as
+// "translation unavailable" — the entry comes back with `Translation
+// == nil` and the caller renders the original body.
+func (s *Service) GetMessage(ctx context.Context, userID, messageID uuid.UUID, targetLang string) (InboxEntry, error) {
 	entry, err := s.deps.Store.LoadInboxEntry(ctx, messageID, userID)
 	if err != nil {
 		return InboxEntry{}, err
@@ -116,9 +124,65 @@ func (s *Service) GetMessage(ctx context.Context, userID, messageID uuid.UUID) (
 	if !allowed[entry.Kind] {
 		return InboxEntry{}, ErrNotFound
 	}
+	if tr := s.resolveTranslation(ctx, entry.Message, targetLang); tr != nil {
+		entry.Translation = tr
+	}
 	return entry, nil
 }
 
+// resolveTranslation returns the cached translation for
+// (message, targetLang), lazily computing and persisting one on
+// cache miss. Returns nil when no translation is needed (target is
+// empty, matches `body_lang`, or the message body is itself
+// undetermined) or when the configured translator declares the
+// rendering unavailable.
+func (s *Service) resolveTranslation(ctx context.Context, msg Message, targetLang string) *Translation {
+	if targetLang == "" || targetLang == msg.BodyLang || msg.BodyLang == LangUndetermined {
+		return nil
+	}
+	if existing, err := s.deps.Store.LoadTranslation(ctx, msg.MessageID, targetLang); err == nil {
+		t := existing
+		return &t
+	} else if !errors.Is(err, ErrNotFound) {
+		s.deps.Logger.Warn("load translation failed",
+			zap.String("message_id", msg.MessageID.String()),
+			zap.String("target_lang", targetLang),
+			zap.Error(err))
+		return nil
+	}
+	if s.deps.Translator == nil {
+		return nil
+	}
+	result, err := s.deps.Translator.Translate(ctx, msg.BodyLang, targetLang, msg.Subject, msg.Body)
+	if err != nil {
+		s.deps.Logger.Warn("translator call failed",
+			zap.String("message_id", msg.MessageID.String()),
+			zap.String("target_lang", targetLang),
+			zap.Error(err))
+		return nil
+	}
+	if result.Engine == "" || result.Engine == "noop" {
+		return nil
+	}
+	tr := Translation{
+		TranslationID:    uuid.New(),
+		MessageID:        msg.MessageID,
+		TargetLang:       targetLang,
+		TranslatedSubject: result.Subject,
+		TranslatedBody:   result.Body,
+		Translator:       result.Engine,
+	}
+	stored, err := s.deps.Store.InsertTranslation(ctx, tr)
+	if err != nil {
+		s.deps.Logger.Warn("insert translation failed",
+			zap.String("message_id", msg.MessageID.String()),
+			zap.String("target_lang", targetLang),
+			zap.Error(err))
+		return nil
+	}
+	return &stored
+}
+
 // ListInbox returns every non-deleted message addressed to userID in
 // gameID, newest first. Read state is preserved per entry; the HTTP
 // layer renders both the message and the recipient row. Personal
@@ -126,7 +190,12 @@ func (s *Service) GetMessage(ctx context.Context, userID, messageID uuid.UUID) (
 // member of the game so a kicked player keeps read access to the
 // admin/system explanation of the kick but not to historical
 // player-to-player threads.
-func (s *Service) ListInbox(ctx context.Context, gameID, userID uuid.UUID) ([]InboxEntry, error) {
+//
+// When `targetLang` is non-empty and differs from a row's body
+// language, the function consults the translation cache (without
+// re-translating on miss; the per-message read endpoint owns that
+// path so the bulk listing never blocks on translator I/O).
+func (s *Service) ListInbox(ctx context.Context, gameID, userID uuid.UUID, targetLang string) ([]InboxEntry, error) {
 	entries, err := s.deps.Store.ListInbox(ctx, gameID, userID)
 	if err != nil {
 		return nil, err
@@ -135,18 +204,46 @@ func (s *Service) ListInbox(ctx context.Context, gameID, userID uuid.UUID) ([]In
 	if err != nil {
 		return nil, err
 	}
-	if allowed[KindPersonal] && allowed[KindAdmin] {
-		return entries, nil
-	}
-	out := make([]InboxEntry, 0, len(entries))
-	for _, e := range entries {
-		if allowed[e.Kind] {
-			out = append(out, e)
+	out := entries
+	if !(allowed[KindPersonal] && allowed[KindAdmin]) {
+		out = make([]InboxEntry, 0, len(entries))
+		for _, e := range entries {
+			if allowed[e.Kind] {
+				out = append(out, e)
+			}
 		}
 	}
+	if targetLang == "" {
+		return out, nil
+	}
+	for i := range out {
+		out[i].Translation = s.lookupCachedTranslation(ctx, out[i].Message, targetLang)
+	}
 	return out, nil
 }
 
+// lookupCachedTranslation reads an existing translation row without
+// asking the Translator to compute one. The bulk inbox listing uses
+// this to avoid per-row translator I/O; GetMessage uses the full
+// `resolveTranslation` helper which falls through to the translator
+// on cache miss.
+func (s *Service) lookupCachedTranslation(ctx context.Context, msg Message, targetLang string) *Translation {
+	if targetLang == "" || targetLang == msg.BodyLang || msg.BodyLang == LangUndetermined {
+		return nil
+	}
+	existing, err := s.deps.Store.LoadTranslation(ctx, msg.MessageID, targetLang)
+	if err != nil {
+		if !errors.Is(err, ErrNotFound) {
+			s.deps.Logger.Debug("inbox translation lookup failed",
+				zap.String("message_id", msg.MessageID.String()),
+				zap.Error(err))
+		}
+		return nil
+	}
+	out := existing
+	return &out
+}
+
 // allowedKinds resolves the set of message kinds the caller may read
 // in gameID. An active member can read everything; a former member
 // (status removed or blocked) can read admin-kind only. A user who