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

models/page.go

@@ -52,8 +52,21 @@ type Page struct {
 	ContentUpdatedAt *lagoon.Date                       `gorm:"column:content_updated_at"`
 	ReportedAt       *time.Time                         `gorm:"column:reported_at"`
 	Note             string                             `gorm:"column:note"`
-	CreatedAt        *time.Time                         `gorm:"column:created_at"`
-	UpdatedAt        *time.Time                         `gorm:"column:updated_at"`
+	CreatedAt        *time.Time                         `gorm:"column:created_at;autoCreateTime:false"`
+	UpdatedAt        *time.Time                         `gorm:"column:updated_at;autoUpdateTime:false"`
+}
+
+// BeforeSave stamps UpdatedAt (and CreatedAt of a new row) in UTC on every
+// save, the admin's and the API's. GORM's own timestamps use the process's
+// local time, which a timestamp column stores as the wall clock: on a
+// machine east of UTC that put the page's sitemap lastmod in the future.
+func (p *Page) BeforeSave(*gorm.DB) error {
+	now := time.Now().UTC()
+	if p.CreatedAt == nil || p.CreatedAt.IsZero() {
+		p.CreatedAt = &now
+	}
+	p.UpdatedAt = &now
+	return nil
 }
 
 func (Page) TableName() string { return "golem15_seo_pages" }