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/cmd/backend/main.go

@@ -309,6 +309,10 @@ func run(ctx context.Context) (err error) {
 	runtimeNotifyPublisher.svc = notifSvc
 
 	diplomailStore := diplomail.NewStore(db)
+	diplomailTranslator, err := buildDiplomailTranslator(cfg.Diplomail, logger)
+	if err != nil {
+		return fmt.Errorf("build diplomail translator: %w", err)
+	}
 	diplomailSvc := diplomail.NewService(diplomail.Deps{
 		Store:        diplomailStore,
 		Memberships:  &diplomailMembershipAdapter{lobby: lobbySvc, users: userSvc},
@@ -316,11 +320,12 @@ func run(ctx context.Context) (err error) {
 		Entitlements: &diplomailEntitlementAdapter{users: userSvc},
 		Games:        &diplomailGameAdapter{lobby: lobbySvc},
 		Detector:     detector.New(),
-		Translator:   translator.NewNoop(),
+		Translator:   diplomailTranslator,
 		Config:       cfg.Diplomail,
 		Logger:       logger,
 	})
 	lobbyDiplomailPublisher.svc = diplomailSvc
+	diplomailWorker := diplomail.NewWorker(diplomailSvc)
 	if email := cfg.Notification.AdminEmail; email == "" {
 		logger.Info("notification admin email not configured (BACKEND_NOTIFICATION_ADMIN_EMAIL); admin-channel routes will be skipped")
 	} else {
@@ -398,7 +403,7 @@ func run(ctx context.Context) (err error) {
 	runtimeScheduler := runtimeSvc.SchedulerComponent()
 	runtimeReconciler := runtimeSvc.Reconciler()
 
-	components := []app.Component{httpServer, pushServer, mailWorker, notifWorker, lobbySweeper, runtimeWorkers, runtimeScheduler, runtimeReconciler}
+	components := []app.Component{httpServer, pushServer, mailWorker, notifWorker, diplomailWorker, lobbySweeper, runtimeWorkers, runtimeScheduler, runtimeReconciler}
 	if metricsServer.Enabled() {
 		components = append(components, metricsServer)
 	}
@@ -641,11 +646,12 @@ func (a *diplomailMembershipAdapter) GetActiveMembership(ctx context.Context, ga
 		return diplomail.ActiveMembership{}, err
 	}
 	return diplomail.ActiveMembership{
-		UserID:   userID,
-		GameID:   gameID,
-		GameName: game.GameName,
-		UserName: account.UserName,
-		RaceName: found.RaceName,
+		UserID:            userID,
+		GameID:            gameID,
+		GameName:          game.GameName,
+		UserName:          account.UserName,
+		RaceName:          found.RaceName,
+		PreferredLanguage: account.PreferredLanguage,
 	}, nil
 }
 
@@ -677,12 +683,13 @@ func (a *diplomailMembershipAdapter) GetMembershipAnyStatus(ctx context.Context,
 		return diplomail.MemberSnapshot{}, err
 	}
 	return diplomail.MemberSnapshot{
-		UserID:   userID,
-		GameID:   gameID,
-		GameName: game.GameName,
-		UserName: account.UserName,
-		RaceName: found.RaceName,
-		Status:   found.Status,
+		UserID:            userID,
+		GameID:            gameID,
+		GameName:          game.GameName,
+		UserName:          account.UserName,
+		RaceName:          found.RaceName,
+		Status:            found.Status,
+		PreferredLanguage: account.PreferredLanguage,
 	}, nil
 }
 
@@ -720,12 +727,13 @@ func (a *diplomailMembershipAdapter) ListMembers(ctx context.Context, gameID uui
 			return nil, fmt.Errorf("resolve user_name for %s: %w", m.UserID, err)
 		}
 		out = append(out, diplomail.MemberSnapshot{
-			UserID:   m.UserID,
-			GameID:   gameID,
-			GameName: game.GameName,
-			UserName: account.UserName,
-			RaceName: m.RaceName,
-			Status:   m.Status,
+			UserID:            m.UserID,
+			GameID:            gameID,
+			GameName:          game.GameName,
+			UserName:          account.UserName,
+			RaceName:          m.RaceName,
+			Status:            m.Status,
+			PreferredLanguage: account.PreferredLanguage,
 		})
 	}
 	return out, nil
@@ -754,6 +762,22 @@ func (a *lobbyDiplomailPublisherAdapter) PublishLifecycle(ctx context.Context, e
 	})
 }
 
+// buildDiplomailTranslator selects the diplomail translator backend
+// from configuration: a non-empty `TranslatorURL` constructs the
+// LibreTranslate HTTP client; an empty URL falls through to the
+// noop translator so deployments without a translation service still
+// boot and deliver mail with the fallback path.
+func buildDiplomailTranslator(cfg config.DiplomailConfig, logger *zap.Logger) (translator.Translator, error) {
+	if cfg.TranslatorURL == "" {
+		logger.Info("diplomail translator URL not configured, using noop translator")
+		return translator.NewNoop(), nil
+	}
+	return translator.NewLibreTranslate(translator.LibreTranslateConfig{
+		URL:     cfg.TranslatorURL,
+		Timeout: cfg.TranslatorTimeout,
+	})
+}
+
 // diplomailEntitlementAdapter implements
 // `diplomail.EntitlementReader` by reading the user-service
 // entitlement snapshot. The IsPaid flag mirrors the per-tier policy