feat(outbox): implement concurrent claim mechanism with UPDATE RETURNING + SKIP LOCKED

- 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

internal/service/platformdelivery/worker_test.go

@@ -64,6 +64,10 @@ func (s *stubEventStore) MarkDeadLetter(_ context.Context, eventID string, attem
 	return nil
 }
 
+func (s *stubEventStore) ReleaseStaleClaims(_ context.Context, _ time.Duration) (int, error) {
+	return 0, nil
+}
+
 func TestWorker_ShouldDeliverPendingEventToCallbackServer(t *testing.T) {
 	now := time.Now().UTC().Truncate(time.Second)
 	store := &stubEventStore{