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/README.md

@@ -17,7 +17,7 @@ purge, and the language-detection / translation cache.
 | A | Schema, personal single-recipient send / read / delete, unread badge, push event with body-language `und` | shipped |
 | B | Owner / admin sends + lifecycle hooks (paused, cancelled, kick); strict soft-access for kicked players | shipped |
 | C | Paid-tier personal broadcast + admin multi-game broadcast + bulk purge + admin observability | shipped |
-| D | Body-language detection (whatlanggo) + translation cache + async worker | planned |
+| D | Body-language detection (whatlanggo) + translation cache + lazy per-read translator dispatch | shipped |
 
 ## Tables
 
@@ -67,8 +67,32 @@ mail to every active member; `Service.changeMembershipStatus` /
   `BACKEND_DIPLOMAIL_MAX_SUBJECT_BYTES` (default 256). Both limits
   live in the service layer so they can be tuned without a schema
   migration.
-- `body_lang` is stored as the BCP 47 `und` (undetermined) sentinel
-  until Stage D wires the auto-detector.
+- `body_lang` is filled at send time by the configured
+  `detector.LanguageDetector` (default: `whatlanggo`, body-only,
+  ≥ 25 runes; shorter bodies stay `und`).
+
+## Translation
+
+Stage D adds a lazy translation cache. When a recipient reads a
+message through `GET /api/v1/user/games/{game_id}/mail/messages/{id}`,
+the handler resolves the caller's `accounts.preferred_language` and
+asks `Service.GetMessage(…, targetLang)` to attach a translation:
+
+- on cache hit (row in `diplomail_translations`), the rendering is
+  returned directly under `translated_subject` / `translated_body`;
+- on cache miss, the configured `translator.Translator` is invoked.
+  A non-noop result is persisted and returned to the caller; the
+  noop translator that ships with Stage D returns `engine == "noop"`,
+  which is treated as "translation unavailable" and the caller falls
+  back to the original body.
+
+The inbox listing (`/inbox`) reuses cached translations but never
+calls the translator on miss — bulk listings stay fast even when a
+real translator (LibreTranslate, SaaS engine) introduces I/O cost.
+
+Future work plugs a real `translator.Translator` (LibreTranslate
+HTTP client is the documented next step) without touching the rest
+of the system.
 
 ## Push integration