feat: add read-only retention inventory

internal/storage/mysql/operational_metrics_integration_test.go

@@ -98,4 +98,15 @@ func TestIntegrationOperationalSnapshot(t *testing.T) {
 	if _, err := ReadOperationalSnapshot(cancelled, db); err == nil {
 		t.Fatal("cancelled context unexpectedly completed snapshot")
 	}
+	f.exec("UPDATE pulse_tasks SET completed_at=DATE_SUB(UTC_TIMESTAMP(6), INTERVAL 2 HOUR), updated_at=UTC_TIMESTAMP(6) WHERE namespace_id=? AND id=?", f.resources.NamespaceID, task.ID)
+	inventory, err := ReadRetentionInventory(ctx, db, time.Now().UTC().Add(-time.Hour))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if inventory.Cutoff.IsZero() || inventory.TerminalTasks == 0 || inventory.UncertainTasks != 0 {
+		t.Fatalf("retention inventory=%+v", inventory)
+	}
+	if _, err := ReadRetentionInventory(ctx, db, time.Now().UTC().Add(time.Hour)); err == nil {
+		t.Fatal("future retention cutoff unexpectedly accepted")
+	}
 }