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/admin/users.html

@@ -65,12 +65,14 @@
                                     {% if user.id != current_user.id %}
                                     <form method="POST" action="{{ url_for('admin_reset_password', user_id=user.id) }}"
                                           class="d-inline" onsubmit="return confirm('Reset password for {{ user.username }}?')">
+                                        <input type="hidden" name="csrf_token" value="{{ csrf_token() }}"/>
                                         <button type="submit" class="btn btn-sm btn-outline-warning" title="Reset Password">
                                             <i class="bi bi-key"></i>
                                         </button>
                                     </form>
                                     <form method="POST" action="{{ url_for('admin_delete_user', user_id=user.id) }}"
                                           class="d-inline" onsubmit="return confirm('Delete user {{ user.username }}? This cannot be undone.')">
+                                        <input type="hidden" name="csrf_token" value="{{ csrf_token() }}"/>
                                         <button type="submit" class="btn btn-sm btn-outline-danger" title="Delete User">
                                             <i class="bi bi-trash"></i>
                                         </button>