Stop emitting no-op docker.updateStatus change rows

dockerUpdateStatusChanged compared the whole DockerUpdateStatusMeta with
reflect.DeepEqual, so the LastChecked stamp refreshed by every periodic
update check emitted a resource_changes row per container per cycle and
flooded unified_resources.db (discussion #1577, same class as #1496).
Only availability, digests, and error text count as change now.

internal/unifiedresources/change_emission.go

@@ -255,14 +255,24 @@ func dockerCommandChanged(before, after Resource) bool {
 }
 
 func dockerUpdateStatusChanged(before, after Resource) bool {
-	var beforeStatus, afterStatus any
+	var beforeStatus, afterStatus *DockerUpdateStatusMeta
 	if before.Docker != nil {
 		beforeStatus = before.Docker.UpdateStatus
 	}
 	if after.Docker != nil {
 		afterStatus = after.Docker.UpdateStatus
 	}
-	return !reflect.DeepEqual(beforeStatus, afterStatus)
+	if beforeStatus == nil || afterStatus == nil {
+		return beforeStatus != afterStatus
+	}
+	// LastChecked is restamped by every periodic update check, so comparing the
+	// whole struct emitted a no-op docker.updateStatus change row per container
+	// per check cycle (the resource_changes flooding behind issue #1577). Only
+	// semantic fields count as change.
+	return beforeStatus.UpdateAvailable != afterStatus.UpdateAvailable ||
+		beforeStatus.CurrentDigest != afterStatus.CurrentDigest ||
+		beforeStatus.LatestDigest != afterStatus.LatestDigest ||
+		beforeStatus.Error != afterStatus.Error
 }
 
 func dockerRestartChanged(before, after Resource) bool {

internal/unifiedresources/change_emission_test.go

@@ -276,6 +276,47 @@ func TestBuildResourceChange_ClassifiesDockerRestartChange(t *testing.T) {
 	}
 }
 
+func TestBuildResourceChange_IgnoresUpdateStatusLastCheckedOnly(t *testing.T) {
+	before := Resource{
+		ID:     "container:1",
+		Type:   ResourceTypeAppContainer,
+		Name:   "container-1",
+		Status: StatusOnline,
+		Docker: &DockerData{
+			Image: "example/app:1",
+			UpdateStatus: &DockerUpdateStatusMeta{
+				UpdateAvailable: false,
+				CurrentDigest:   "sha256:aaa",
+				LatestDigest:    "sha256:aaa",
+				LastChecked:     time.Date(2026, 7, 14, 6, 0, 0, 0, time.UTC),
+			},
+		},
+	}
+	after := before
+	refreshed := *before.Docker.UpdateStatus
+	refreshed.LastChecked = refreshed.LastChecked.Add(2 * time.Hour)
+	afterDocker := *before.Docker
+	afterDocker.UpdateStatus = &refreshed
+	after.Docker = &afterDocker
+
+	if change := buildResourceChange(before, true, after, true, time.Now().UTC(), nil, SourcePulseDiff, ""); change != nil {
+		t.Fatalf("expected no change for LastChecked-only refresh, got %+v", change)
+	}
+
+	available := refreshed
+	available.UpdateAvailable = true
+	available.LatestDigest = "sha256:bbb"
+	afterDocker.UpdateStatus = &available
+
+	change := buildResourceChange(before, true, after, true, time.Now().UTC(), nil, SourcePulseDiff, "")
+	if change == nil {
+		t.Fatal("expected update-status change when digest/availability changed, got nil")
+	}
+	if !sameStringSet(mustChangedFields(t, change), []string{"docker.updateStatus"}) {
+		t.Fatalf("changedFields = %+v, want docker.updateStatus", mustChangedFields(t, change))
+	}
+}
+
 func TestBuildResourceChange_ClassifiesKubernetesRestartChange(t *testing.T) {
 	before := Resource{
 		ID:     "pod:1",