fix(sqlite): concurrent migrate retries

docs/adr/0015-transactional-outbox.md

@@ -79,7 +79,9 @@ backends dead-letter after the same number of attempts.
   and updates `duplicate_window` (1 h) and the consumer's `max_deliver`/`ack_wait` in place, with a
   warning in the log. Both are mutable on a live stream/consumer, so no recreate is needed;
   immutable consumer fields (name, filter, deliver policy) are never touched.
-- Starting server and worker at the same moment against a database that still needs migration `0101`
-  can make one fail with `duplicate column name`; start the server first, or restart the loser.
+- Server and worker both migrate at startup (docker compose starts them together). `sqlite::migrate`
+  retries with a bounded backoff (100 ms to 1 s, 5 retries), so the loser of a migration race
+  re-reads `_sqlx_migrations`, skips what the winner applied and succeeds; only a persistent error
+  is returned.
 - Handler authors must keep new handlers idempotent; the ledger narrows duplicates but cannot
   remove the crash-after-effect case.