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/openapi.yaml

@@ -4352,6 +4352,24 @@ components:
           type: string
           format: date-time
           nullable: true
+        translated_subject:
+          type: string
+          description: |
+            Subject rendered into the caller's preferred_language by
+            the translation cache. Absent when the caller's language
+            matches `body_lang` or the translator could not produce
+            a rendering.
+        translated_body:
+          type: string
+          description: |
+            Body rendered into the caller's preferred_language. Same
+            absence semantics as `translated_subject`.
+        translation_lang:
+          type: string
+          description: BCP 47 tag of the rendered translation.
+        translator:
+          type: string
+          description: Identifier of the translation engine that produced the cached row.
     UserMailSentSummary:
       type: object
       additionalProperties: false