feat(catchup): arm on hello, fire missed-window backups on tick

internal/server/http/host_credentials.go

@@ -483,6 +483,12 @@ func (s *Server) onAgentHello(ctx context.Context, hostID string, conn *ws.Conn)
 	// and the drain may take seconds across many rows. A non-blocking
 	// goroutine keeps the hello path snappy.
 	go s.DrainPending(context.Background(), hostID)
+	// Intermittent hosts that just reconnected may have slept through a
+	// backup window. Arm a catch-up evaluation after a settle delay; the
+	// pending-drain tick fires it. Always-on hosts never need this.
+	if host, err := s.deps.Store.GetHost(ctx, hostID); err == nil && !host.AlwaysOn {
+		s.ArmCatchup(hostID, time.Now().UTC())
+	}
 }
 
 // maybeAutoInit dispatches a `restic init` job iff the host has no