feat: page registry with its admin screen and signpost type

- golem15_seo_pages (migration 202610280002): the site's own frontend pages
  with an address and a name per locale, kind, origin, parent, content date
  and note; settings routes_reported_at
- SEO > Pages (golem15.seo.pages) after URL rules: list with kind, origin
  and missing filters and the missing row state, form with the P9 partial;
  the plugin's form extension saves the address and name per locale and
  refuses url_required, url_invalid, url_taken and url_owned
- menu item type seo-page with a content descriptor, so each page gets
  the SEO tab, resolves ok and is listed in the sitemaps grouped by kind
  (pages, offers, services, landing-pages, titled per locale)
- management API GET/POST /_seo/api/v1/pages, PUT/DELETE /pages/{id} and
  the MCP tools seo_list_pages, seo_create_page, seo_update_page,
  seo_delete_page (in this commit because the operations parity test maps
  every Pages operation to its tool)

models/settings.go

@@ -95,8 +95,11 @@ type Settings struct {
 	VerifyOther        lagoon.Jsonable[[]VerifyMeta] `gorm:"column:verify_other"`
 	VerifyFiles        lagoon.Jsonable[[]VerifyFile] `gorm:"column:verify_files"`
 	CSVDelimiter       string                        `gorm:"column:csv_delimiter"`
-	CreatedAt          *time.Time                    `gorm:"column:created_at"`
-	UpdatedAt          *time.Time                    `gorm:"column:updated_at"`
+	// RoutesReportedAt is the time of the newest route report (D-33),
+	// written by the report only.
+	RoutesReportedAt *time.Time `gorm:"column:routes_reported_at"`
+	CreatedAt        *time.Time `gorm:"column:created_at"`
+	UpdatedAt        *time.Time `gorm:"column:updated_at"`
 }
 
 func (Settings) TableName() string { return "golem15_seo_settings" }