Store "interrupted by restart" for jobs recovered at boot

Boot recovery now writes the required fixed text to the job error column,
and the recovery test asserts that literal text instead of the constant.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/jobs/jobs_test.go

@@ -391,8 +391,8 @@ func TestRestartFailsInterruptedJobsAndRunsQueuedOnes(t *testing.T) {
 	start(t, second)
 
 	job := waitFor(t, second, interrupted, finished)
-	if job.Status != StatusFailed || job.Error == nil || *job.Error != CodeInterrupted {
-		t.Errorf("interrupted job = %+v, want failed with %q", job, CodeInterrupted)
+	if job.Status != StatusFailed || job.Error == nil || *job.Error != "interrupted by restart" {
+		t.Errorf("interrupted job = %+v, want failed with %q", job, "interrupted by restart")
 	}
 	if string(job.Result) != `{"done":1}` {
 		t.Errorf("interrupted job result = %s, want the last partial result kept", job.Result)