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/deps.go

@@ -5,6 +5,8 @@ import (
 	"time"
 
 	"galaxy/backend/internal/config"
+	"galaxy/backend/internal/diplomail/detector"
+	"galaxy/backend/internal/diplomail/translator"
 
 	"github.com/google/uuid"
 	"go.uber.org/zap"
@@ -25,6 +27,8 @@ type Deps struct {
 	Notification NotificationPublisher
 	Entitlements EntitlementReader
 	Games        GameLookup
+	Detector     detector.LanguageDetector
+	Translator   translator.Translator
 	Config       config.DiplomailConfig
 	Logger       *zap.Logger
 	Now          func() time.Time