nats nak w/ shared retry_backoff, exhausted log; no 'will retry' lie

crates/domain/src/events.rs

@@ -155,6 +155,14 @@ pub trait AckHandle: Send + Sync {
     async fn defer(&self, delay: std::time::Duration) -> Result<(), DomainError>;
 }
 
+/// Wait before retrying an event whose `attempt`-th delivery just failed:
+/// 5s doubling per attempt, capped at 5 minutes. Every backend (SQLite queue,
+/// JetStream nak, outbox relay) uses this one schedule.
+pub fn retry_backoff(attempt: u32) -> std::time::Duration {
+    let secs = 5u64 * (1u64 << attempt.min(6));
+    std::time::Duration::from_secs(secs.min(300))
+}
+
 /// Stable identity of one queued event; the key for de-duplicating deliveries.
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
 pub struct EventId(uuid::Uuid);