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/store.go

@@ -358,6 +358,71 @@ func (s *Store) UnreadCountForUserGame(ctx context.Context, gameID, userID uuid.
 	return int(dest.Count), nil
 }
 
+// translationColumns is the canonical projection for
+// diplomail_translations reads.
+func translationColumns() postgres.ColumnList {
+	t := table.DiplomailTranslations
+	return postgres.ColumnList{
+		t.TranslationID, t.MessageID, t.TargetLang,
+		t.TranslatedSubject, t.TranslatedBody, t.Translator, t.TranslatedAt,
+	}
+}
+
+// LoadTranslation returns the cached translation row for
+// (messageID, targetLang). Returns ErrNotFound when no cache row
+// exists yet — the caller decides whether to compute and persist
+// one.
+func (s *Store) LoadTranslation(ctx context.Context, messageID uuid.UUID, targetLang string) (Translation, error) {
+	t := table.DiplomailTranslations
+	stmt := postgres.SELECT(translationColumns()).
+		FROM(t).
+		WHERE(t.MessageID.EQ(postgres.UUID(messageID)).
+			AND(t.TargetLang.EQ(postgres.String(targetLang)))).
+		LIMIT(1)
+	var row model.DiplomailTranslations
+	if err := stmt.QueryContext(ctx, s.db, &row); err != nil {
+		if errors.Is(err, qrm.ErrNoRows) {
+			return Translation{}, ErrNotFound
+		}
+		return Translation{}, fmt.Errorf("diplomail store: load translation %s/%s: %w", messageID, targetLang, err)
+	}
+	return translationFromModel(row), nil
+}
+
+// InsertTranslation persists a new translation cache row. The unique
+// constraint on (message_id, target_lang) prevents duplicate
+// renderings. Callers that race on the same (message, lang) pair
+// should be prepared for a UNIQUE violation; the second writer can
+// fall back to LoadTranslation.
+func (s *Store) InsertTranslation(ctx context.Context, in Translation) (Translation, error) {
+	t := table.DiplomailTranslations
+	stmt := t.INSERT(
+		t.TranslationID, t.MessageID, t.TargetLang,
+		t.TranslatedSubject, t.TranslatedBody, t.Translator,
+	).VALUES(
+		in.TranslationID, in.MessageID, in.TargetLang,
+		in.TranslatedSubject, in.TranslatedBody, in.Translator,
+	).RETURNING(translationColumns())
+
+	var row model.DiplomailTranslations
+	if err := stmt.QueryContext(ctx, s.db, &row); err != nil {
+		return Translation{}, fmt.Errorf("diplomail store: insert translation %s/%s: %w", in.MessageID, in.TargetLang, err)
+	}
+	return translationFromModel(row), nil
+}
+
+func translationFromModel(row model.DiplomailTranslations) Translation {
+	return Translation{
+		TranslationID:    row.TranslationID,
+		MessageID:        row.MessageID,
+		TargetLang:       row.TargetLang,
+		TranslatedSubject: row.TranslatedSubject,
+		TranslatedBody:   row.TranslatedBody,
+		Translator:       row.Translator,
+		TranslatedAt:     row.TranslatedAt,
+	}
+}
+
 // DeleteMessagesForGames removes every diplomail_messages row whose
 // game_id falls in the supplied set. The cascade defined on the
 // `diplomail_recipients` and `diplomail_translations` foreign keys