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/postgres/jet/backend/model/diplomail_recipients.go

@@ -13,14 +13,18 @@ import (
 )
 
 type DiplomailRecipients struct {
-	RecipientID       uuid.UUID `sql:"primary_key"`
-	MessageID         uuid.UUID
-	GameID            uuid.UUID
-	UserID            uuid.UUID
-	RecipientUserName string
-	RecipientRaceName *string
-	DeliveredAt       *time.Time
-	ReadAt            *time.Time
-	DeletedAt         *time.Time
-	NotifiedAt        *time.Time
+	RecipientID                uuid.UUID `sql:"primary_key"`
+	MessageID                  uuid.UUID
+	GameID                     uuid.UUID
+	UserID                     uuid.UUID
+	RecipientUserName          string
+	RecipientRaceName          *string
+	RecipientPreferredLanguage string
+	AvailableAt                *time.Time
+	TranslationAttempts        int32
+	NextTranslationAttemptAt   *time.Time
+	DeliveredAt                *time.Time
+	ReadAt                     *time.Time
+	DeletedAt                  *time.Time
+	NotifiedAt                 *time.Time
 }