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/cmd/backend/main.go

@@ -27,6 +27,8 @@ import (
 	"galaxy/backend/internal/config"
 	"galaxy/backend/internal/devsandbox"
 	"galaxy/backend/internal/diplomail"
+	"galaxy/backend/internal/diplomail/detector"
+	"galaxy/backend/internal/diplomail/translator"
 	"galaxy/backend/internal/dockerclient"
 	"galaxy/backend/internal/engineclient"
 	"galaxy/backend/internal/geo"
@@ -313,6 +315,8 @@ func run(ctx context.Context) (err error) {
 		Notification: &diplomailNotificationPublisherAdapter{svc: notifSvc},
 		Entitlements: &diplomailEntitlementAdapter{users: userSvc},
 		Games:        &diplomailGameAdapter{lobby: lobbySvc},
+		Detector:     detector.New(),
+		Translator:   translator.NewNoop(),
 		Config:       cfg.Diplomail,
 		Logger:       logger,
 	})