feat: photo-search, system settings, ROADMAP

- PhotoSearchModal: Yandex photo-search с профилями доменов
- SystemSettings: управление app_settings (admin-only, /system)
- ROADMAP.md: актуальный план фич P1-P10
- Header, ChannelView, session: поддержка is_admin

lib/session.js

@@ -20,5 +20,11 @@ export async function getSession() {
 export async function requireUser() {
   const s = await getSession();
   if (!s.userId) return null;
-  return { id: s.userId, email: s.email, name: s.name };
+  return { id: s.userId, email: s.email, name: s.name, isAdmin: !!s.isAdmin };
+}
+
+export async function requireAdmin() {
+  const u = await requireUser();
+  if (!u || !u.isAdmin) return null;
+  return u;
 }