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)

updates/202610280002_create_golem15_seo_pages.go

@@ -0,0 +1,56 @@
+package updates
+
+import (
+	"github.com/go-gormigrate/gormigrate/v2"
+	"gorm.io/gorm"
+)
+
+// createPages creates golem15_seo_pages, the page registry (D-33): the
+// site's own frontend pages with their address per locale (url is the
+// default locale's, urls the others'), name per locale, kind, origin,
+// parent, content date, last report and note, one row per default-locale
+// address. The settings get routes_reported_at, the time of the newest
+// route report, against which a reported page shows as missing.
+var createPages = []*gormigrate.Migration{
+	{
+		ID: "202610280002_create_golem15_seo_pages",
+		Migrate: func(tx *gorm.DB) error {
+			if err := tx.Exec(`CREATE TABLE golem15_seo_pages (
+	id BIGSERIAL PRIMARY KEY,
+	url TEXT NOT NULL,
+	urls JSONB NOT NULL DEFAULT '{}'::jsonb,
+	title TEXT NOT NULL DEFAULT '',
+	titles JSONB NOT NULL DEFAULT '{}'::jsonb,
+	kind TEXT NOT NULL DEFAULT 'page',
+	origin TEXT NOT NULL DEFAULT 'manual',
+	parent_id BIGINT NULL REFERENCES golem15_seo_pages (id) ON DELETE SET NULL,
+	content_updated_at DATE NULL,
+	reported_at TIMESTAMPTZ NULL,
+	note TEXT NOT NULL DEFAULT '',
+	created_at TIMESTAMP(0) NULL,
+	updated_at TIMESTAMP(0) NULL
+)`).Error; err != nil {
+				return err
+			}
+			// One page per default-locale address; a row being created holds
+			// '' only inside its own save transaction.
+			if err := tx.Exec(`CREATE UNIQUE INDEX golem15_seo_pages_url ON golem15_seo_pages (url) WHERE url <> ''`).Error; err != nil {
+				return err
+			}
+			if err := tx.Exec(`CREATE INDEX golem15_seo_pages_urls ON golem15_seo_pages USING gin (urls jsonb_path_ops)`).Error; err != nil {
+				return err
+			}
+			return tx.Exec(`ALTER TABLE golem15_seo_settings ADD COLUMN routes_reported_at TIMESTAMPTZ NULL`).Error
+		},
+		Rollback: func(tx *gorm.DB) error {
+			if err := tx.Exec(`ALTER TABLE golem15_seo_settings DROP COLUMN IF EXISTS routes_reported_at`).Error; err != nil {
+				return err
+			}
+			return tx.Exec(`DROP TABLE IF EXISTS golem15_seo_pages`).Error
+		},
+	},
+}
+
+func init() {
+	Register(createPages...)
+}