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/diplomail.go

@@ -22,6 +22,8 @@ import (
 	"time"
 
 	"galaxy/backend/internal/config"
+	"galaxy/backend/internal/diplomail/detector"
+	"galaxy/backend/internal/diplomail/translator"
 
 	"go.uber.org/zap"
 )
@@ -91,6 +93,12 @@ func NewService(deps Deps) *Service {
 	if deps.Notification == nil {
 		deps.Notification = NewNoopNotificationPublisher(deps.Logger)
 	}
+	if deps.Detector == nil {
+		deps.Detector = detector.NoopDetector{}
+	}
+	if deps.Translator == nil {
+		deps.Translator = translator.NewNoop()
+	}
 	if deps.Config.MaxBodyBytes <= 0 {
 		deps.Config.MaxBodyBytes = 4096
 	}