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/api/client.js

@@ -108,6 +108,8 @@ export const api = {
     getSettings: () => req('/admin/settings'),
     updateSettings: (obj) => req('/admin/settings', { method: 'PUT', body: obj }),
     activity: (limit = 50) => req(`/admin/activity?limit=${limit}`),
+    botActivity: () => req('/admin/bot-activity'),
+    unbanIp: (ip) => req('/admin/bot-activity/unban', { method: 'POST', body: { ip } }),
     listUsers: () => req('/admin/users'),
     createUser: (data) => req('/admin/users', { method: 'POST', body: data }),
     updateUser: (id, data) => req(`/admin/users/${id}`, { method: 'PUT', body: data }),