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/config/config.go

@@ -98,6 +98,10 @@ const (
 
 	envDiplomailMaxBodyBytes = "BACKEND_DIPLOMAIL_MAX_BODY_BYTES"
 	envDiplomailMaxSubjectBytes = "BACKEND_DIPLOMAIL_MAX_SUBJECT_BYTES"
+	envDiplomailTranslatorURL = "BACKEND_DIPLOMAIL_TRANSLATOR_URL"
+	envDiplomailTranslatorTimeout = "BACKEND_DIPLOMAIL_TRANSLATOR_TIMEOUT"
+	envDiplomailTranslatorMaxAttempts = "BACKEND_DIPLOMAIL_TRANSLATOR_MAX_ATTEMPTS"
+	envDiplomailWorkerInterval = "BACKEND_DIPLOMAIL_WORKER_INTERVAL"
 
 	envDevSandboxEmail = "BACKEND_DEV_SANDBOX_EMAIL"
 	envDevSandboxEngineImage = "BACKEND_DEV_SANDBOX_ENGINE_IMAGE"
@@ -168,6 +172,9 @@ const (
 
 	defaultDiplomailMaxBodyBytes = 4096
 	defaultDiplomailMaxSubjectBytes = 256
+	defaultDiplomailTranslatorTimeout = 10 * time.Second
+	defaultDiplomailTranslatorMaxAttempts = 5
+	defaultDiplomailWorkerInterval = 2 * time.Second
 
 	defaultDevSandboxEngineVersion = "0.1.0"
 	defaultDevSandboxPlayerCount = 20
@@ -418,6 +425,26 @@ type DiplomailConfig struct {
 	// in bytes. Subjects are optional; the empty-string default
 	// passes the limit trivially.
 	MaxSubjectBytes int
+
+	// TranslatorURL is the base URL of the LibreTranslate-compatible
+	// instance the async translation worker calls. When empty, the
+	// worker still runs but falls through to "deliver original"
+	// (the noop translator returns engine=noop).
+	TranslatorURL string
+
+	// TranslatorTimeout bounds a single HTTP request to the
+	// translator. Worker retries (exponential backoff up to
+	// TranslatorMaxAttempts) layer on top.
+	TranslatorTimeout time.Duration
+
+	// TranslatorMaxAttempts is the number of times the worker tries
+	// to translate one (message, target_lang) pair before falling
+	// back to delivering the original body.
+	TranslatorMaxAttempts int
+
+	// WorkerInterval bounds how often the async translation worker
+	// scans for pending pairs. The worker handles one pair per tick.
+	WorkerInterval time.Duration
 }
 
 // NotificationConfig configures the notification fan-out module
@@ -520,6 +547,9 @@ func DefaultConfig() Config {
 		Diplomail: DiplomailConfig{
 			MaxBodyBytes: defaultDiplomailMaxBodyBytes,
 			MaxSubjectBytes: defaultDiplomailMaxSubjectBytes,
+			TranslatorTimeout: defaultDiplomailTranslatorTimeout,
+			TranslatorMaxAttempts: defaultDiplomailTranslatorMaxAttempts,
+			WorkerInterval: defaultDiplomailWorkerInterval,
 		},
 		DevSandbox: DevSandboxConfig{
 			EngineVersion: defaultDevSandboxEngineVersion,
@@ -690,6 +720,16 @@ func LoadFromEnv() (Config, error) {
 	if cfg.Diplomail.MaxSubjectBytes, err = loadInt(envDiplomailMaxSubjectBytes, cfg.Diplomail.MaxSubjectBytes); err != nil {
 		return Config{}, err
 	}
+	cfg.Diplomail.TranslatorURL = loadString(envDiplomailTranslatorURL, cfg.Diplomail.TranslatorURL)
+	if cfg.Diplomail.TranslatorTimeout, err = loadDuration(envDiplomailTranslatorTimeout, cfg.Diplomail.TranslatorTimeout); err != nil {
+		return Config{}, err
+	}
+	if cfg.Diplomail.TranslatorMaxAttempts, err = loadInt(envDiplomailTranslatorMaxAttempts, cfg.Diplomail.TranslatorMaxAttempts); err != nil {
+		return Config{}, err
+	}
+	if cfg.Diplomail.WorkerInterval, err = loadDuration(envDiplomailWorkerInterval, cfg.Diplomail.WorkerInterval); err != nil {
+		return Config{}, err
+	}
 
 	cfg.DevSandbox.Email = strings.TrimSpace(loadString(envDevSandboxEmail, cfg.DevSandbox.Email))
 	cfg.DevSandbox.EngineImage = strings.TrimSpace(loadString(envDevSandboxEngineImage, cfg.DevSandbox.EngineImage))
@@ -894,6 +934,15 @@ func (c Config) Validate() error {
 	if c.Diplomail.MaxSubjectBytes < 0 {
 		return fmt.Errorf("%s must not be negative", envDiplomailMaxSubjectBytes)
 	}
+	if c.Diplomail.TranslatorTimeout <= 0 {
+		return fmt.Errorf("%s must be positive", envDiplomailTranslatorTimeout)
+	}
+	if c.Diplomail.TranslatorMaxAttempts <= 0 {
+		return fmt.Errorf("%s must be positive", envDiplomailTranslatorMaxAttempts)
+	}
+	if c.Diplomail.WorkerInterval <= 0 {
+		return fmt.Errorf("%s must be positive", envDiplomailWorkerInterval)
+	}
 	if email := strings.TrimSpace(c.Notification.AdminEmail); email != "" {
 		if _, err := netmail.ParseAddress(email); err != nil {
 			return fmt.Errorf("%s must be a valid RFC 5322 address: %w", envNotificationAdminEmail, err)