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>

.env.example

@@ -1,5 +1,7 @@
 # .env にコピーして値を設定する（chmod 600 推奨）
 ACCESS_KEY=000000
+# 管理者キー（任意・ACCESS_KEY と別の6桁）。入力すると管理者として入室し、ハンドレンジ学習ページが見える
+# ADMIN_KEY=000001
 PORT=4671
 # HOST=127.0.0.1
 # 本番のみ設定（ローカル開発では設定しない）