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/deps.go

@@ -77,14 +77,15 @@ type GameSnapshot struct {
 // ActiveMembership is the slim view of a single (user, game) roster
 // row the diplomail package needs at send time: it confirms the
 // participant is active in the game and captures the snapshot fields
-// (`game_name`, `user_name`, `race_name`) that we persist on each new
-// message / recipient row.
+// (`game_name`, `user_name`, `race_name`, `preferred_language`) that
+// we persist on each new message / recipient row.
 type ActiveMembership struct {
-	UserID   uuid.UUID
-	GameID   uuid.UUID
-	GameName string
-	UserName string
-	RaceName string
+	UserID            uuid.UUID
+	GameID            uuid.UUID
+	GameName          string
+	UserName          string
+	RaceName          string
+	PreferredLanguage string
 }
 
 // MembershipLookup is the read-only surface diplomail uses to verify
@@ -123,14 +124,17 @@ const (
 // MemberSnapshot is the slim view of a membership row that survives
 // all three status values. RaceName is the immutable string captured
 // at registration time; an empty value is legal for rare cases where
-// the row was inserted without one.
+// the row was inserted without one. PreferredLanguage is included so
+// the broadcast and lifecycle paths can decide whether the recipient
+// needs to wait for a translation before delivery.
 type MemberSnapshot struct {
-	UserID   uuid.UUID
-	GameID   uuid.UUID
-	GameName string
-	UserName string
-	RaceName string
-	Status   string
+	UserID            uuid.UUID
+	GameID            uuid.UUID
+	GameName          string
+	UserName          string
+	RaceName          string
+	PreferredLanguage string
+	Status            string
 }
 
 // NotificationPublisher is the outbound surface diplomail uses to