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/translator/translator.go

@@ -0,0 +1,59 @@
+// Package translator wraps the per-language rendering for the
+// diplomail subsystem. The package exposes a narrow `Translator`
+// interface so the actual translation backend (LibreTranslate, an
+// in-process model, a SaaS engine, …) can be swapped without
+// touching the rest of the codebase.
+//
+// Stage D ships a `NoopTranslator` that returns the input unchanged.
+// The diplomail Service treats a `Name == NoopEngine` result as
+// "translation unavailable" and refrains from writing a cache row;
+// the inbox handler then returns the original body with a
+// `translated == false` payload. The contract lets the rest of the
+// system ship without a translation backend; future stages can wire
+// a real `Translator` without code changes elsewhere.
+package translator
+
+import "context"
+
+// NoopEngine is the engine identifier returned by `NoopTranslator`.
+// The diplomail Service checks for this value to decide whether to
+// persist a `diplomail_translations` row.
+const NoopEngine = "noop"
+
+// Result carries one translated rendering plus the engine identifier
+// that produced it. The engine name is persisted as
+// `diplomail_translations.translator` so an operator can see which
+// backend produced each row.
+type Result struct {
+	Subject string
+	Body    string
+	Engine  string
+}
+
+// Translator is the read-only surface diplomail consumes when it
+// needs to render a message for a recipient whose
+// `preferred_language` differs from `body_lang`. Implementations
+// must be safe for concurrent use; `Translate` may be invoked from
+// the async worker on many messages at once.
+type Translator interface {
+	// Translate renders `subject` and `body` from `srcLang` into
+	// `dstLang`. A nil error with `Result.Engine == NoopEngine`
+	// signals that no real rendering happened.
+	Translate(ctx context.Context, srcLang, dstLang, subject, body string) (Result, error)
+}
+
+// NewNoop returns a Translator that always returns the input
+// unchanged with engine name `NoopEngine`.
+func NewNoop() Translator {
+	return noop{}
+}
+
+type noop struct{}
+
+func (noop) Translate(_ context.Context, _, _, subject, body string) (Result, error) {
+	return Result{
+		Subject: subject,
+		Body:    body,
+		Engine:  NoopEngine,
+	}, nil
+}