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)

extensions_pages.go

@@ -0,0 +1,142 @@
+package seo
+
+import (
+	"context"
+	"sort"
+
+	"git.golem15.com/golem15/sm-seo-plugin/classes"
+	"git.golem15.com/golem15/sm-seo-plugin/controllers"
+	"git.golem15.com/golem15/sm-seo-plugin/models"
+	"git.golem15.com/golem15/summercms/modules/pact"
+)
+
+// pagesFormYAML adds the translatable address and name to the Pages form
+// (UI-SPEC C21): url after page_status, title after kind.
+const pagesFormYAML = `fields:
+    url:
+        label: golem15.seo::lang.pages.url
+        comment: golem15.seo::lang.pages.url_comment
+        type: mltext
+        span: left
+        after: page_status
+    title:
+        label: golem15.seo::lang.pages.title
+        comment: golem15.seo::lang.pages.title_comment
+        type: mltext
+        span: left
+        after: kind
+`
+
+// pagesForm is the plugin's form extension on its own Pages controller:
+// it loads and saves a page's address and name per locale (the default
+// locale in the url and title columns, the others in urls and titles),
+// refuses a page with the C21 copy (url_required, url_invalid, url_taken,
+// url_owned) inside the save transaction, and records the save in
+// history. It is listed before the SEO tab, so the tab's address tracking
+// sees the saved address.
+func (p *Plugin) pagesForm() *pact.AdminFormExtension {
+	return &pact.AdminFormExtension{
+		FieldsYAML:  []byte(pagesFormYAML),
+		Permissions: []string{controllers.PermissionManage},
+		Load: func(ctx context.Context, in pact.AdminFormLoad) (map[string]any, error) {
+			svc := p.Service()
+			if svc == nil {
+				return nil, classes.ErrNotBooted
+			}
+			page, ok := in.Record.(*models.Page)
+			if !ok || page == nil {
+				return map[string]any{}, nil
+			}
+			db := in.DB.WithContext(ctx)
+			def := svc.DefaultLocale(ctx, db)
+			urls := map[string]string{}
+			titles := map[string]string{}
+			for _, locale := range svc.Locales(ctx, db) {
+				urls[locale], titles[locale] = "", ""
+			}
+			for locale, path := range page.LocaleURLs(def) {
+				urls[locale] = path
+			}
+			titles[def] = page.Title
+			for locale, title := range page.Titles.Get() {
+				if locale != def {
+					titles[locale] = title
+				}
+			}
+			return map[string]any{"url": urls, "title": titles}, nil
+		},
+		Save: func(ctx context.Context, in pact.AdminFormSave) (pact.AdminFormSaveResult, error) {
+			svc := p.Service()
+			if svc == nil {
+				return pact.AdminFormSaveResult{}, classes.ErrNotBooted
+			}
+			page, ok := in.Record.(*models.Page)
+			if !ok || page == nil {
+				return pact.AdminFormSaveResult{}, nil
+			}
+			var before map[string]any
+			if !in.Created {
+				before = classes.PageValues(*page)
+				old, _ := classes.ChangesValues(in.Changes, false)
+				for k, v := range old {
+					before[k] = v
+				}
+			}
+			urls, urlsSent := localeTexts(in.Values["url"])
+			titles, titlesSent := localeTexts(in.Values["title"])
+			if !urlsSent {
+				urls = nil
+			}
+			if !titlesSent {
+				titles = nil
+			}
+			if err := svc.ApplyPageTexts(ctx, in.DB, page, urls, titles); err != nil {
+				return pact.AdminFormSaveResult{}, err
+			}
+			problems, err := svc.CheckPage(ctx, in.DB, *page)
+			if err != nil {
+				return pact.AdminFormSaveResult{}, err
+			}
+			if list := problems["url"]; len(list) > 0 {
+				refusals := pact.AdminFormErrors{}
+				for _, pr := range list {
+					refusals["url"] = append(refusals["url"], svc.PhraseCtx(ctx, pr.Key, pr.Params))
+				}
+				return pact.AdminFormSaveResult{}, refusals
+			}
+			if err := in.DB.Model(&models.Page{}).Where("id = ?", page.ID).Updates(map[string]any{
+				"url": page.URL, "urls": page.URLs, "title": page.Title, "titles": page.Titles,
+			}).Error; err != nil {
+				return pact.AdminFormSaveResult{}, err
+			}
+			return pact.AdminFormSaveResult{}, svc.RecordPage(ctx, in.DB, before, *page, false)
+		},
+	}
+}
+
+// localeTexts reads a submitted translatable value as locale -> text;
+// sent is false when the request did not send the field. A plain string
+// is the default locale's ("" key).
+func localeTexts(raw any) (map[string]string, bool) {
+	switch v := raw.(type) {
+	case nil:
+		return nil, false
+	case map[string]string:
+		return v, true
+	case map[string]any:
+		out := make(map[string]string, len(v))
+		keys := make([]string, 0, len(v))
+		for k := range v {
+			keys = append(keys, k)
+		}
+		sort.Strings(keys)
+		for _, k := range keys {
+			s, _ := v[k].(string)
+			out[k] = s
+		}
+		return out, true
+	case string:
+		return map[string]string{"": v}, true
+	}
+	return nil, false
+}