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/admin_send.go

@@ -157,7 +157,7 @@ func (s *Service) SendPlayerBroadcast(ctx context.Context, in SendPlayerBroadcas
 		SenderIP:       in.SenderIP,
 		Subject:        subject,
 		Body:           body,
-		BodyLang:       LangUndetermined,
+		BodyLang:       s.deps.Detector.Detect(body),
 		BroadcastScope: BroadcastScopeGameBroadcast,
 	}
 	rcptInserts := make([]RecipientInsert, 0, len(members))
@@ -429,7 +429,7 @@ func (s *Service) buildAdminMessageInsert(callerKind string, callerUserID *uuid.
 		SenderIP:       senderIP,
 		Subject:        subject,
 		Body:           body,
-		BodyLang:       LangUndetermined,
+		BodyLang:       s.deps.Detector.Detect(body),
 		BroadcastScope: scope,
 	}
 	switch callerKind {