diplomail (Stage E): LibreTranslate client + async translation worker

Synchronous translation on read (Stage D) blocks the HTTP handler on
translator I/O. Stage E switches to "send moments-fast, deliver
when translated": recipients whose preferred_language differs from
the detected body_lang are inserted with available_at=NULL, and an
async worker turns them on once a LibreTranslate call materialises
the cache row (or fails terminally after 5 retries).

Schema delta on diplomail_recipients: available_at,
translation_attempts, next_translation_attempt_at, plus a snapshot
recipient_preferred_language so the worker queries do not need a
join. Read paths (ListInbox, GetMessage, UnreadCount) filter on
available_at IS NOT NULL. Push fan-out is moved from Service to the
worker so the recipient only sees the toast when the inbox row is
actually visible.

Translator backend is now a configurable choice: empty
BACKEND_DIPLOMAIL_TRANSLATOR_URL → noop (deliver original);
populated → LibreTranslate HTTP client. Per-attempt timeout, max
attempts, and worker interval all live in DiplomailConfig. The HTTP
client itself is unit-tested via httptest (happy path, BCP47
normalisation, unsupported pair, 5xx, identical src/dst, missing
URL); worker delivery + fallback paths are covered by the
testcontainers-backed e2e tests in diplomail_e2e_test.go.

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

backend/internal/diplomail/service.go

@@ -72,18 +72,20 @@ func (s *Service) SendPersonal(ctx context.Context, in SendPersonalInput) (Messa
 		BroadcastScope: BroadcastScopeSingle,
 	}
 	raceName := recipient.RaceName
-	var raceNamePtr *string
-	if raceName != "" {
-		raceNamePtr = &raceName
-	}
-	rcptInsert := RecipientInsert{
-		RecipientID:       uuid.New(),
-		MessageID:         msgInsert.MessageID,
-		GameID:            in.GameID,
-		UserID:            in.RecipientUserID,
-		RecipientUserName: recipient.UserName,
-		RecipientRaceName: raceNamePtr,
-	}
+	rcptInsert := buildRecipientInsert(
+		msgInsert.MessageID,
+		MemberSnapshot{
+			UserID:            in.RecipientUserID,
+			GameID:            in.GameID,
+			GameName:          recipient.GameName,
+			UserName:          recipient.UserName,
+			RaceName:          raceName,
+			PreferredLanguage: recipient.PreferredLanguage,
+			Status:            "active",
+		},
+		msgInsert.BodyLang,
+		s.nowUTC(),
+	)
 
 	msg, recipients, err := s.deps.Store.InsertMessageWithRecipients(ctx, msgInsert, []RecipientInsert{rcptInsert})
 	if err != nil {
@@ -93,7 +95,9 @@ func (s *Service) SendPersonal(ctx context.Context, in SendPersonalInput) (Messa
 		return Message{}, Recipient{}, fmt.Errorf("diplomail: send personal: unexpected recipient count %d", len(recipients))
 	}
 
-	s.publishMessageReceived(ctx, msg, recipients[0])
+	if recipients[0].AvailableAt != nil {
+		s.publishMessageReceived(ctx, msg, recipients[0])
+	}
 	return msg, recipients[0], nil
 }