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)

settings_test.go

@@ -25,11 +25,13 @@ func tabsOf(fields []schemaField) []string {
 // General and Indexing, their values in golem15_seo_type_settings with the
 // D-21 defaults, template checks, and the inject switch hiding the SEO tab.
 func TestSettingsTypeTabs(t *testing.T) {
+	// The SEO plugin's own page registry (seo-page, "Site page") is a
+	// content type too, registered after the fixture's.
 	t.Run("three content types", func(t *testing.T) {
 		env := bootEnv(t)
 		token := env.admin(t, true)
 		fields := env.formSchema(t, token, "golem15/seo/settings")
-		if got, want := strings.Join(tabsOf(fields), " | "), "General | Page | Tags | Notes | Indexing | Addresses | Verification"; got != want {
+		if got, want := strings.Join(tabsOf(fields), " | "), "General | Page | Tags | Notes | Site page | Indexing | Addresses | Verification"; got != want {
 			t.Fatalf("settings tabs = %q, want %q", got, want)
 		}
 		findField(t, fields, "type_acme_page_inject")
@@ -76,10 +78,10 @@ func TestSettingsTypeTabs(t *testing.T) {
 		}
 	})
 
-	t.Run("no content types", func(t *testing.T) {
+	t.Run("no other content types", func(t *testing.T) {
 		env := bootPlugins(t, []string{"golem15.seo"})
 		token := env.admin(t, true)
-		if got, want := strings.Join(tabsOf(env.formSchema(t, token, "golem15/seo/settings")), " | "), "General | Indexing | Addresses | Verification"; got != want {
+		if got, want := strings.Join(tabsOf(env.formSchema(t, token, "golem15/seo/settings")), " | "), "General | Site page | Indexing | Addresses | Verification"; got != want {
 			t.Fatalf("settings tabs = %q, want %q", got, want)
 		}
 	})