refactor(alert): refresh stale_schedule docs; log tick schedule errors; add mode-change + never-backed-up tests

internal/alert/engine.go

@@ -196,11 +196,9 @@ func (e *Engine) handleHostOnline(ctx context.Context, hostID string) {
 // tick is the 60-second sweep. Responsibilities:
 //  1. Re-evaluate agent_offline for every offline host that may have
 //     crossed the floor between explicit events.
-//  2. Stale-schedule detection — declared in the spec but intentionally
-//     left as a no-op in v1. The precise "expected to have fired but
-//     didn't" trigger requires a store helper that lands in a later
-//     task. The KindStaleSchedule constant is exported so UI code can
-//     reference the tag string today.
+//  2. Stale-schedule detection for intermittent hosts — raises
+//     stale_schedule when LastBackupAt is older than 7 days and the
+//     host has an enabled schedule. Always-on hosts are excluded.
 func (e *Engine) tick(ctx context.Context, now time.Time) {
 	// User-management cleanup piggy-backed here for now. Setup tokens
 	// have a 1h expiry; the alert engine tick is the cheapest existing
@@ -232,7 +230,11 @@ func (e *Engine) tick(ctx context.Context, now time.Time) {
 				continue
 			}
 			hasEnabled, err := e.hostHasEnabledSchedule(ctx, h.ID)
-			if err != nil || !hasEnabled {
+			if err != nil {
+				slog.Warn("alert: tick list schedules", "host_id", h.ID, "err", err)
+				continue
+			}
+			if !hasEnabled {
 				continue
 			}
 			e.raiseAndNotify(ctx, h.ID, KindStaleSchedule, "", "warning",

internal/alert/intermittent_test.go

@@ -170,3 +170,86 @@ func TestNoStalenessWithoutEnabledSchedule(t *testing.T) {
 		}
 	}
 }
+
+// TestResolveOnModeChangeClearsOfflineAlert checks that ResolveOnModeChange
+// clears an open agent_offline alert when a host's mode is toggled.
+func TestResolveOnModeChangeClearsOfflineAlert(t *testing.T) {
+	t.Parallel()
+	eng, st, hostID := setupEngine(t)
+	ctx := context.Background()
+
+	// Make always-on and set it offline with a stale last_seen_at.
+	if err := st.SetHostAlwaysOn(ctx, hostID, true); err != nil {
+		t.Fatalf("SetHostAlwaysOn: %v", err)
+	}
+	if _, err := st.DB().Exec(
+		`UPDATE hosts SET last_seen_at = ?, status = ? WHERE id = ?`,
+		time.Now().UTC().Add(-2*time.Hour).Format(time.RFC3339Nano),
+		"offline",
+		hostID,
+	); err != nil {
+		t.Fatalf("update last_seen_at: %v", err)
+	}
+
+	// Raise the offline alert.
+	eng.handleHostOffline(ctx, hostID)
+
+	open, _ := st.ListAlerts(ctx, store.AlertFilter{Status: "open", HostID: hostID})
+	if len(open) != 1 || open[0].Kind != KindAgentOffline {
+		t.Fatalf("expected 1 agent_offline alert before mode change; got %d: %+v", len(open), open)
+	}
+
+	// Toggle mode — should clear the alert.
+	eng.ResolveOnModeChange(ctx, hostID, time.Now().UTC())
+
+	open, _ = st.ListAlerts(ctx, store.AlertFilter{Status: "open", HostID: hostID})
+	for _, a := range open {
+		if a.Kind == KindAgentOffline {
+			t.Fatalf("expected agent_offline to be resolved after mode change; still open: %+v", a)
+		}
+	}
+}
+
+// TestNoStalenessWhenNeverBackedUp checks that no stale_schedule alert is
+// raised for an intermittent host that has never backed up (nil LastBackupAt).
+func TestNoStalenessWhenNeverBackedUp(t *testing.T) {
+	t.Parallel()
+	eng, st, hostID := setupEngine(t)
+	ctx := context.Background()
+
+	// Make intermittent.
+	if err := st.SetHostAlwaysOn(ctx, hostID, false); err != nil {
+		t.Fatalf("SetHostAlwaysOn: %v", err)
+	}
+
+	// Create a source group and an enabled schedule — but do NOT set LastBackupAt.
+	sgID := ulid.Make().String()
+	if err := st.CreateSourceGroup(ctx, &store.SourceGroup{
+		ID:       sgID,
+		HostID:   hostID,
+		Name:     "default",
+		Includes: []string{"/home"},
+	}); err != nil {
+		t.Fatalf("CreateSourceGroup: %v", err)
+	}
+
+	schedID := ulid.Make().String()
+	if err := st.CreateSchedule(ctx, &store.Schedule{
+		ID:             schedID,
+		HostID:         hostID,
+		CronExpr:       "0 2 * * *",
+		Enabled:        true,
+		SourceGroupIDs: []string{sgID},
+	}); err != nil {
+		t.Fatalf("CreateSchedule: %v", err)
+	}
+
+	eng.tick(ctx, time.Now().UTC())
+
+	open, _ := st.ListAlerts(ctx, store.AlertFilter{Status: "open", HostID: hostID})
+	for _, a := range open {
+		if a.Kind == KindStaleSchedule {
+			t.Fatalf("expected no stale_schedule when never backed up; got: %+v", a)
+		}
+	}
+}

internal/alert/rules.go

@@ -27,10 +27,10 @@ const (
 	// integrity is at risk) when a check job fails.
 	KindCheckFailed = "check_failed"
 
-	// KindStaleSchedule is declared for completeness but intentionally
-	// left as a no-op in v1. The precise "expected to have fired but
-	// didn't" logic requires a store helper that lands in a follow-up
-	// task. Ask the team before implementing.
+	// KindStaleSchedule is raised for intermittent (non-always-on) hosts
+	// when their last successful backup is older than staleBackupThreshold
+	// (7 days) and they have at least one enabled schedule. Resolved on
+	// backup success or when the host is switched to always-on mode.
 	KindStaleSchedule = "stale_schedule"
 
 	// KindAgentOffline is raised when a host's last_seen_at is older