Add admin role with an admin-only hand range study page

- Optional ADMIN_KEY (6 digits, distinct from ACCESS_KEY): entering it on the
  key screen signs in as admin (role-specific HMAC cookie; member cookies
  stay valid). Logged-in members can open /key?admin to enter it.
- /range (range chart + color/category quiz, ported from poker-hand and
  restyled to the site design) and its files under admin/ are served only to
  admins; everyone else gets 404. Card images cached privately for a day.
- Lobby shows a link to the page for admins only (GET /api/me).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/assets/key.js

@@ -8,6 +8,12 @@
   const submitBtn = document.getElementById('submit');
   let busy = false;
 
+  // /key?admin … 入室済みの人が管理者キーを入れるための画面
+  if (new URLSearchParams(location.search).has('admin')) {
+    document.getElementById('key-title').textContent = '管理者キーを入力';
+    document.getElementById('key-hint').textContent = '管理者用の6桁の数字';
+  }
+
   const value = () => inputs.map((i) => i.value).join('');
 
   function fill(digits, from) {