[management] Fix peer deletion error handling (#5188)

When a deleted peer tries to reconnect, GetUserIDByPeerKey was returning
Internal error instead of NotFound, causing clients to retry indefinitely
instead of recognizing the unrecoverable PermissionDenied error.

This fix:
1. Updates GetUserIDByPeerKey to properly return NotFound when peer doesn't exist
2. Updates Sync handler to convert NotFound to PermissionDenied with message
   'peer is not registered', matching the behavior of GetAccountIDForPeerKey

Fixes the regression introduced in v0.61.1 where deleted peers would see:
- Before: 'rpc error: code = Internal desc = failed handling request' (retry loop)
- After: 'rpc error: code = PermissionDenied desc = peer is not registered' (exits)

management/internals/shared/grpc/server.go

@@ -232,6 +232,9 @@ func (s *Server) Sync(req *proto.EncryptedMessage, srv proto.ManagementService_S
 	userID, err := s.accountManager.GetUserIDByPeerKey(ctx, peerKey.String())
 	if err != nil {
 		s.syncSem.Add(-1)
+		if errStatus, ok := internalStatus.FromError(err); ok && errStatus.Type() == internalStatus.NotFound {
+			return status.Errorf(codes.PermissionDenied, "peer is not registered")
+		}
 		return mapError(ctx, err)
 	}