feat: register the Users admin screen with the privileged-member guard

- permissions (the four Winter codes and manage_privileged_groups),
  the user navigation item and the embedded admin YAML
- Users controller: list with deactivated users, row states, the
  groups, created date and activated filters, and the form's name,
  surname and email
- a member of a group in golem15.user.privileged_groups (default
  admin) keeps their email, password and row unless the administrator
  holds golem15.users.manage_privileged_groups; the refusal is a 403
  that saves nothing

admin.go

@@ -0,0 +1,36 @@
+package user
+
+import (
+	"embed"
+	"io/fs"
+
+	"git.golem15.com/golem15/sm-user-plugin/controllers"
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/pact"
+)
+
+// adminFS holds the admin YAML and partials, file by file: the controllers
+// directory also holds files that are not admin assets.
+//
+//go:embed controllers/users/config_list.yaml controllers/users/config_form.yaml controllers/users/config_filter.yaml models/user/fields.yaml models/user/columns.yaml
+var adminFS embed.FS
+
+func (p *Plugin) AdminFS() fs.FS { return adminFS }
+
+// AdminControllers hands the controllers a lazy accessor of the booted app,
+// so they read the database handle and the config at request time.
+func (p *Plugin) AdminControllers() []pact.AdminController {
+	return controllers.AdminControllers(func() *backpack.App {
+		if p == nil {
+			return nil
+		}
+		return p.app
+	})
+}
+
+var (
+	_ pact.AdminAssets         = (*Plugin)(nil)
+	_ pact.HasAdminControllers = (*Plugin)(nil)
+	_ pact.HasPermissions      = (*Plugin)(nil)
+	_ pact.HasNavigation       = (*Plugin)(nil)
+)