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/page/columns.yaml

@@ -0,0 +1,29 @@
+# Pages list columns (UI-SPEC C21). kind and origin show their labels (the
+# controller's ListExtendRecords) and sort by the stored values.
+columns:
+    url:
+        label: golem15.seo::lang.pages.url
+        type: text
+        searchable: true
+        sortable: true
+    title:
+        label: golem15.seo::lang.pages.title
+        type: text
+        searchable: true
+    kind:
+        label: golem15.seo::lang.pages.kind
+        type: text
+        sortable: true
+    origin:
+        label: golem15.seo::lang.pages.origin
+        type: text
+        sortable: true
+    content_updated_at:
+        label: golem15.seo::lang.pages.content_updated
+        type: date
+        sortable: true
+    reported_at:
+        label: golem15.seo::lang.pages.reported
+        type: datetime
+        sortable: true
+        invisible: true

models/page/fields.yaml

@@ -0,0 +1,43 @@
+# Page form (UI-SPEC C21). url (after page_status) and title (after kind)
+# are translatable and added by the plugin's form extension, which stores
+# them per locale; the SEO tab follows.
+fields:
+    page_status:
+        type: partial
+        path: page_status
+        span: full
+        context: update
+
+    kind:
+        label: golem15.seo::lang.pages.kind
+        comment: golem15.seo::lang.pages.kind_comment
+        type: dropdown
+        span: right
+        default: page
+        options:
+            page: golem15.seo::lang.pages.kind_page
+            offer: golem15.seo::lang.pages.kind_offer
+            service: golem15.seo::lang.pages.kind_service
+            landing: golem15.seo::lang.pages.kind_landing
+
+    parent_id:
+        label: golem15.seo::lang.pages.parent
+        comment: golem15.seo::lang.pages.parent_comment
+        type: dropdown
+        span: right
+        emptyOption: golem15.seo::lang.pages.parent_none
+        options: parentOptions
+
+    content_updated_at:
+        label: golem15.seo::lang.pages.content_updated
+        comment: golem15.seo::lang.pages.content_updated_comment
+        type: datepicker
+        mode: date
+        span: left
+
+    note:
+        label: golem15.seo::lang.pages.note
+        comment: golem15.seo::lang.pages.note_comment
+        type: textarea
+        size: tiny
+        span: full