fix: relay shutdown, claim lease, dead-letter undecodable, dedup window

crates/adapters/sqlite/migrations/0101_outbox_identity.sql

@@ -2,6 +2,9 @@
 -- (event_id, handler) pairs so redeliveries are skipped.
 ALTER TABLE event_queue ADD COLUMN event_id TEXT;
 
+-- When the current claim started; a claim older than the lease is reclaimable.
+ALTER TABLE event_queue ADD COLUMN claimed_at TEXT;
+
 -- Legacy rows get a random v4-shaped uuid. The subquery references the row id
 -- so it is re-evaluated per row rather than cached.
 UPDATE event_queue SET event_id = (