diplomail (Stage E): LibreTranslate client + async translation worker

Synchronous translation on read (Stage D) blocks the HTTP handler on
translator I/O. Stage E switches to "send moments-fast, deliver
when translated": recipients whose preferred_language differs from
the detected body_lang are inserted with available_at=NULL, and an
async worker turns them on once a LibreTranslate call materialises
the cache row (or fails terminally after 5 retries).

Schema delta on diplomail_recipients: available_at,
translation_attempts, next_translation_attempt_at, plus a snapshot
recipient_preferred_language so the worker queries do not need a
join. Read paths (ListInbox, GetMessage, UnreadCount) filter on
available_at IS NOT NULL. Push fan-out is moved from Service to the
worker so the recipient only sees the toast when the inbox row is
actually visible.

Translator backend is now a configurable choice: empty
BACKEND_DIPLOMAIL_TRANSLATOR_URL → noop (deliver original);
populated → LibreTranslate HTTP client. Per-attempt timeout, max
attempts, and worker interval all live in DiplomailConfig. The HTTP
client itself is unit-tested via httptest (happy path, BCP47
normalisation, unsupported pair, 5xx, identical src/dst, missing
URL); worker delivery + fallback paths are covered by the
testcontainers-backed e2e tests in diplomail_e2e_test.go.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/internal/diplomail/README.md

@@ -18,6 +18,7 @@ purge, and the language-detection / translation cache.
 | 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 + lazy per-read translator dispatch | shipped |
+| E | LibreTranslate HTTP client + async translation worker with exponential backoff + delivery gating on translation completion | shipped |
 
 ## Tables
 
@@ -94,6 +95,47 @@ Future work plugs a real `translator.Translator` (LibreTranslate
 HTTP client is the documented next step) without touching the rest
 of the system.
 
+## Async translation (Stage E)
+
+Stage E switches the translation pipeline from "lazy at read" to
+"async at send". The send path stays synchronous from the
+caller's perspective: the message and recipient rows are inserted
+in one transaction. What changes is delivery semantics:
+
+- Recipients whose `preferred_language` matches the detected
+  `body_lang` (or whose body language is `und`) get
+  `available_at = now()` straight away and the push event fires
+  during the request.
+- Recipients whose `preferred_language` differs are inserted with
+  `available_at IS NULL`. They are **not** visible in inbox, unread
+  count, or push events until the worker translates the message.
+
+The worker (`internal/diplomail.Worker`, started as an
+`app.Component` in `cmd/backend/main`) ticks once every
+`BACKEND_DIPLOMAIL_WORKER_INTERVAL` (default `2s`). Each tick:
+
+1. Picks one distinct `(message_id, recipient_preferred_language)`
+   pair from `diplomail_recipients` where `available_at IS NULL`
+   and `next_translation_attempt_at` is unset or due.
+2. Loads the source message, checks the translation cache.
+3. On cache hit → marks every pending recipient of the pair
+   delivered and emits push.
+4. On cache miss → asks the configured `Translator`:
+   - success → caches the translation, marks delivered, push;
+   - HTTP 400 (unsupported pair) → marks delivered without a
+     translation (fallback to original);
+   - other failure → bumps `translation_attempts`, schedules the
+     retry via `next_translation_attempt_at`, leaves pending.
+5. After `BACKEND_DIPLOMAIL_TRANSLATOR_MAX_ATTEMPTS` (default `5`)
+   the worker falls back to delivering the original body so a
+   prolonged LibreTranslate outage does not strand messages.
+
+Retry backoff is exponential `1s → 2s → 4s → 8s → 16s` (capped at
+60s) per pair. Operators monitor the LibreTranslate dependency
+through standard OpenTelemetry export — translation outcomes
+surface in `diplomail.worker` logs at Info / Warn levels;
+Grafana / Prometheus dashboards live outside this package.
+
 ## Push integration
 
 Every successful send emits a `diplomail.message.received` push