Add Bot Activity admin panel: banned-IP view + recent events + emergency unban

Expose the botScore middleware's in-memory scoring/ban state to admins.
Previously state lived only in the store Map with no persistence or API — the
only visibility was tailing container logs.

- botScore: bounded ring buffer (300) recording scan/login-fail/honeypot and
  ban events (most-recent-first); listState() snapshot of all scored IPs;
  unban() to clear a single IP.
- New admin-only endpoints GET /admin/bot-activity and
  POST /admin/bot-activity/unban (RBAC admin gate, IP validated). Unban is
  activity-logged with the admin username.
- Bot Activity tab: currently-banned table with Unban, plus a recent-events
  feed, following the existing admin table patterns.
- Tests for the buffer, listState, and unban (guard lets an unbanned IP back
  through). README updated.

Read + emergency-unban only — no ban-add or weight-editing surface. Buffer is
in-memory, matching the store; not persisted.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/src/routes/admin/AdminLayout.jsx

@@ -11,6 +11,7 @@ const NAV = [
   { to: '/admin/hero', label: 'Hero Editor' },
   { to: '/admin/settings', label: 'Settings' },
   { to: '/admin/activity', label: 'Activity' },
+  { to: '/admin/bot-activity', label: 'Bot Activity' },
   { to: '/admin/users', label: 'Users' },
   { to: '/admin/account', label: 'Account' },
 ]
@@ -22,6 +23,7 @@ const TITLES = {
   '/admin/hero': 'Hero Editor',
   '/admin/settings': 'Site Settings',
   '/admin/activity': 'Activity Log',
+  '/admin/bot-activity': 'Bot Activity',
   '/admin/users': 'Users',
   '/admin/account': 'Account Security',
 }