fix: create fresh context per watermark write retry attempt

Each retry in the SetOutboxWatermark loop now gets its own 5s context.
Previously a shared context could already be expired when the second or
third attempt ran, making the retries pointless.
Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

backend/internal/service/scheduler_snapshot_service.go

@@ -264,11 +264,11 @@ func (s *SchedulerSnapshotService) pollOutbox() {
 	}
 
 	lastID := events[len(events)-1].ID
-	wmCtx, wmCancel := context.WithTimeout(context.Background(), 5*time.Second)
-	defer wmCancel()
 	var wmErr error
 	for i := range 3 {
+		wmCtx, wmCancel := context.WithTimeout(context.Background(), 5*time.Second)
 		wmErr = s.cache.SetOutboxWatermark(wmCtx, lastID)
+		wmCancel()
 		if wmErr == nil {
 			break
 		}