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/key.html

@@ -14,8 +14,8 @@
     <div class="logo">Hold'em<br>Night<small>PRIVATE CLUB</small></div>
 
     <form id="form" class="card" autocomplete="off">
-      <h2 class="key-title">参加キーを入力</h2>
-      <p class="hint key-hint">オーナーから教えてもらった6桁の数字</p>
+      <h2 class="key-title" id="key-title">参加キーを入力</h2>
+      <p class="hint key-hint" id="key-hint">オーナーから教えてもらった6桁の数字</p>
       <div class="key-inputs" id="digits">
         <input type="tel" inputmode="numeric" aria-label="1桁目" autocomplete="one-time-code">
         <input type="tel" inputmode="numeric" aria-label="2桁目">