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)

lang/en/lang.yaml

@@ -2,6 +2,7 @@ plugin:
     name: SEO
     description: Search engine metadata for every content type, URL rules and global SEO settings.
 nav:
+    pages: Pages
     robots: robots.txt
     notfound: 404 log
     redirects: Redirects
@@ -171,6 +172,7 @@ history:
     what_robots: robots.txt
     what_settings: SEO settings
     what_notfound: 404 log entry “:url”
+    what_page: Page “:url”
     change_cell:
         one: ':field: :before → :after'
         other: ':count fields changed'
@@ -202,6 +204,7 @@ history:
     kind_robots: robots.txt
     kind_settings: SEO settings
     kind_notfound: 404 log entry
+    kind_page: registry page
 jobs:
     generate_entries: 'SEO: generate entries'
 effective:
@@ -346,3 +349,46 @@ transfer:
     row_canonical: "The canonical URL redirects to “:target”"
     row_rule: "No record at this address: a URL rule is created"
     row_invalid: ":field: :message"
+pages:
+    type: Site page
+    list_title: Pages
+    name: Page
+    new: New page
+    list_empty: No pages yet. Add the site's own pages here, such as the home page, contact or offer pages, so they get an SEO tab and a place in the sitemaps. The site can also report its pages automatically.
+    url: Address
+    title: Name
+    kind: Type
+    origin: Origin
+    content_updated: Content changed
+    reported: Last reported
+    kind_page: Page
+    kind_offer: Offer
+    kind_service: Service
+    kind_landing: Landing page
+    origin_manual: Manual
+    origin_reported: Reported by the site
+    origin_mcp: MCP
+    state_missing: Not on the site
+    filter_missing: Not on the site
+    filter_missing_only: Only missing
+    filter_missing_hide: Hide missing
+    url_comment: The page's path in each language, e.g. /kontakt. Leave a language empty when the page has no version in it.
+    title_comment: Used for the default SEO title, the HTML sitemap and breadcrumbs.
+    kind_comment: "Decides the sitemap file and the default structured data: offers and services become Service."
+    parent: Parent page
+    parent_none: None (top level)
+    parent_comment: Places the page under another one in breadcrumbs.
+    content_updated_comment: Sent as lastmod in the sitemaps. Change it when the page's content changes on the site.
+    note: Note
+    note_comment: Not shown on the site.
+    status_reported: Reported by the site on :time.
+    status_missing: The site's last report (:time) did not include this address. Check whether the page still exists, then delete it here or redirect it.
+    url_required: Enter the address in the default language.
+    url_invalid: Start the address with / and leave out the domain.
+    url_taken: Another page already uses this address.
+    url_owned: This address belongs to :type “:title”. Set its SEO there.
+    parent_loop: A page cannot be placed under itself or one of its subpages.
+    delete_confirm: Delete this page from the registry? Its SEO tab is deleted with it, and the address leaves the sitemaps. A page the site reports comes back with its next report.
+    delete_selected_confirm:
+        one: "Delete :count page from the registry? Its SEO tab is deleted with it."
+        other: "Delete :count pages from the registry? Their SEO tabs are deleted with them."