release: 1.6.0: 30-day send stats and optional auto level-2 rate limits

30-day sending statistics per domain and application, plus optional auto level-2 rate limits derived from average send rate.
Added:
  - Domain page **Sending statistics** card: total messages, peak and average
  - Level-2 rate limit **Manual** / **Auto** mode on domain and application
  - Domain export JSON now includes rate limits (`mode`, `auto_multiplier`,
Changed:
  - Auto mode uses the level-1 window (`RATE_LIMIT_WINDOW_SECONDS`); manual

Full detail: CHANGELOG.md (1.6.0).

internal/logtail/logtail.go

@@ -43,6 +43,11 @@ var pollInterval = time.Second
 // startup). It is a var so tests can shorten it.
 var retentionInterval = 6 * time.Hour
 
+// RetentionInterval returns how often background send-log pruning runs.
+func RetentionInterval() time.Duration {
+	return retentionInterval
+}
+
 // queueIDs lists the messages Postfix currently holds, for the reconcile sweep.
 // It is a var so tests can answer without a running Postfix.
 var queueIDs = postfix.QueueIDs