fix: store registry page timestamps in UTC

- GORM stamped created_at and updated_at in the process's local time,
  which the timestamp columns keep as the wall clock; east of UTC a page
  without a content date got a sitemap lastmod in the future
- Page.BeforeSave stamps both in UTC for the admin's and the API's saves,
  and the admin's address change writes updated_at in UTC

registry_test.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 	"sync"
 	"testing"
+	"time"
 
 	"git.golem15.com/golem15/sm-sitemap-plugin/contract"
 	"git.golem15.com/golem15/summercms/modules/cooler"
@@ -32,6 +33,58 @@ func (e *testEnv) refusal(t *testing.T, method, path, token string, body map[str
 	}
 }
 
+// TestRegistryTimestampsUTC pins the registry page's created_at and
+// updated_at in UTC on a machine east of UTC: the admin create, the
+// management API's update and the admin's address change each store the
+// UTC wall clock, so a page without a content date never gets a lastmod
+// in the future.
+func TestRegistryTimestampsUTC(t *testing.T) {
+	local := time.Local
+	time.Local = time.FixedZone("UTC+3", 3*60*60)
+	t.Cleanup(func() { time.Local = local })
+	env := bootEnv(t)
+	token := env.admin(t, true)
+
+	stamps := func(id string) (created, updated time.Time) {
+		t.Helper()
+		var row struct {
+			CreatedAt time.Time
+			UpdatedAt time.Time
+		}
+		if err := env.db.Raw(`SELECT created_at, updated_at FROM golem15_seo_pages WHERE id = ?`, id).Scan(&row).Error; err != nil {
+			t.Fatal(err)
+		}
+		return row.CreatedAt, row.UpdatedAt
+	}
+	within := func(what string, got, from, to time.Time) {
+		t.Helper()
+		if got.Before(from.Add(-time.Second)) || got.After(to.Add(time.Second)) {
+			t.Fatalf("%s = %s, want between %s and %s (UTC)", what, got.Format(time.RFC3339), from.Format(time.RFC3339), to.Format(time.RFC3339))
+		}
+	}
+
+	from := time.Now().UTC()
+	id := env.createRegistryPage(t, token, map[string]any{"url": map[string]any{"en": "/kontakt"}, "title": map[string]any{"en": "Kontakt"}})
+	created, updated := stamps(id)
+	within("created_at after the admin create", created, from, time.Now().UTC())
+	within("updated_at after the admin create", updated, from, time.Now().UTC())
+
+	from = time.Now().UTC()
+	env.expect(t, http.StatusOK, http.MethodPut, "/_seo/api/v1/pages/"+id, token, map[string]any{"title": "Kontakt z nami", "content_updated_at": ""})
+	_, updated = stamps(id)
+	within("updated_at after the API update", updated, from, time.Now().UTC())
+	reg, _ := env.app.Lookup[*signpost.Registry]()
+	rec, err := reg.Describe(t.Context(), "seo-page", id, "")
+	if err != nil || rec == nil || rec.LastMod == nil || rec.LastMod.After(time.Now().UTC().Add(time.Second)) {
+		t.Fatalf("describe = %+v err=%v, want a lastmod no later than now", rec, err)
+	}
+
+	from = time.Now().UTC()
+	env.expect(t, http.StatusOK, http.MethodPut, pagesAPI+"/"+id, token, map[string]any{"url": map[string]any{"en": "/kontakt-z-nami"}})
+	_, updated = stamps(id)
+	within("updated_at after the admin address change", updated, from, time.Now().UTC())
+}
+
 // TestRegistry covers the page registry (D-33, UI-SPEC C21): a page made
 // in the Pages screen is a seo-page record with the SEO tab, resolves ok,
 // is listed in the sitemap file of its kind with hreflang alternates, the