Detect and alert on stuck jobs

internal/server/metrics/metrics_test.go

@@ -114,6 +114,8 @@ func TestRenderGolden(t *testing.T) {
 	snap := r.SnapshotWith(hosts,
 		map[string]int{"info": 0, "warning": 1, "critical": 0},
 		"v1.2.3", "deadbeef", "go1.25.0")
+	snap.StuckJobs = 2
+	snap.OldestStuckAge = 90 * time.Minute
 
 	var buf bytes.Buffer
 	if err := Render(&buf, snap); err != nil {
@@ -129,6 +131,8 @@ func TestRenderGolden(t *testing.T) {
 		`rm_active_alerts{severity="info"} 0`,
 		`rm_active_alerts{severity="critical"} 0`,
 		`rm_build_info{version="v1.2.3",commit="deadbeef",go_version="go1.25.0"} 1`,
+		`rm_stuck_jobs 2`,
+		`rm_oldest_stuck_job_age_seconds 5400`,
 		`rm_host_agent_online{host_id="01H0001",host="alpha"} 1`,
 		`rm_host_agent_online{host_id="01H0002",host="bravo"} 0`,
 		`rm_host_last_backup_timestamp_seconds{host_id="01H0001",host="alpha"} 1700000000`,