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/migrations/00001_init.sql

@@ -722,16 +722,20 @@ CREATE INDEX diplomail_messages_sender_user_idx
 -- rare admin notifications addressed to a player who no longer has an
 -- active membership in the game.
 CREATE TABLE diplomail_recipients (
-    recipient_id          uuid        PRIMARY KEY,
-    message_id            uuid        NOT NULL REFERENCES diplomail_messages (message_id) ON DELETE CASCADE,
-    game_id               uuid        NOT NULL,
-    user_id               uuid        NOT NULL,
-    recipient_user_name   text        NOT NULL,
-    recipient_race_name   text,
-    delivered_at          timestamptz,
-    read_at               timestamptz,
-    deleted_at            timestamptz,
-    notified_at           timestamptz,
+    recipient_id                  uuid        PRIMARY KEY,
+    message_id                    uuid        NOT NULL REFERENCES diplomail_messages (message_id) ON DELETE CASCADE,
+    game_id                       uuid        NOT NULL,
+    user_id                       uuid        NOT NULL,
+    recipient_user_name           text        NOT NULL,
+    recipient_race_name           text,
+    recipient_preferred_language  text        NOT NULL DEFAULT '',
+    available_at                  timestamptz,
+    translation_attempts          integer     NOT NULL DEFAULT 0,
+    next_translation_attempt_at   timestamptz,
+    delivered_at                  timestamptz,
+    read_at                       timestamptz,
+    deleted_at                    timestamptz,
+    notified_at                   timestamptz,
     CONSTRAINT diplomail_recipients_unique UNIQUE (message_id, user_id)
 );
 
@@ -740,7 +744,17 @@ CREATE INDEX diplomail_recipients_inbox_idx
 
 CREATE INDEX diplomail_recipients_unread_idx
     ON diplomail_recipients (user_id, game_id)
-    WHERE read_at IS NULL AND deleted_at IS NULL;
+    WHERE read_at IS NULL AND deleted_at IS NULL AND available_at IS NOT NULL;
+
+-- Index drives the translation worker's pending-pair pickup. The
+-- partial filter keeps the scan tight: terminal-state recipients
+-- (with a non-NULL available_at) never appear in this btree. The
+-- composite ordering puts the next-attempt clock first so the
+-- backoff filter (`next_translation_attempt_at <= now()`) seeks
+-- before the secondary cluster on (message_id, lang).
+CREATE INDEX diplomail_recipients_pending_translation_idx
+    ON diplomail_recipients (next_translation_attempt_at, message_id, recipient_preferred_language)
+    WHERE available_at IS NULL;
 
 -- diplomail_translations caches one rendered translation per
 -- (message, target_lang) so a broadcast addressed to many recipients