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/account.html

@@ -157,6 +157,7 @@
                             <form method="POST" action="{{ url_for('account_delete_key', key_id=key.id) }}"
                                   style="display:inline;"
                                   onsubmit="return confirm('Delete key &quot;{{ key.name }}&quot;?')">
+                                <input type="hidden" name="csrf_token" value="{{ csrf_token() }}"/>
                                 <button type="submit" class="btn btn-outline-danger" title="Delete">
                                     <i class="bi bi-trash"></i>
                                 </button>