Fix 6 security issues from post-FR audit

- Fix 3 missing CSRF tokens on admin user delete/reset and account
  key delete forms (were broken — CSRFProtect rejected submissions)
- Fix trust store path traversal: untrust_key() now validates
  fingerprint format ([0-9a-f]{32}) and checks resolved path
- Fix chain key rotation: old key is now revoked after rotation
  record, preventing compromised old keys from appending records
- Fix SSRF in deadman webhook: block private/internal IP targets
- Fix logout CSRF: /logout is now POST-only with CSRF token,
  preventing cross-site forced logout via img tags

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontends/web/templates/base.html

@@ -102,7 +102,7 @@
                                 {% endif %}
                                 <li><hr class="dropdown-divider"></li>
                                 <li><a class="dropdown-item" href="/keys"><i class="bi bi-key me-2"></i>Keys</a></li>
-                                <li><a class="dropdown-item" href="/logout"><i class="bi bi-box-arrow-left me-2"></i>Logout</a></li>
+                                <li><form method="POST" action="/logout" class="d-inline"><input type="hidden" name="csrf_token" value="{{ csrf_token() }}"/><button type="submit" class="dropdown-item"><i class="bi bi-box-arrow-left me-2"></i>Logout</button></form></li>
                             </ul>
                         </li>
                         {% else %}