- Add migration 0004 to introduce 'claiming' status and timeout index
- Add StatusClaiming to platformevent domain and allow it in Validate()
- Rewrite ListDue as transactional UPDATE ... RETURNING with FOR UPDATE SKIP LOCKED
- Add ReleaseStaleClaims to reset expired claiming events back to retrying
- Worker Start() now runs a 30s ticker for stale claim recovery (5m timeout)
- Update stubEventStore in tests to satisfy new EventStore interface
Refs: D-02
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