feat(gui): add reverse DNS hostname detection for peers

Resolve hostname via PTR record on peer endpoint IP.
Results cached in memory for 5 minutes to avoid latency.
Hostname shown below endpoint in the peers table.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gui/templates/index.html

@@ -40,7 +40,10 @@
           </td>
           <td class="mono-sm">{{ p.client_address }}</td>
           <td class="mono-sm text-muted">{{ p.routes }}</td>
-          <td class="mono-sm text-muted">{{ p.endpoint }}</td>
+          <td>
+            <span class="mono-sm text-muted">{{ p.endpoint }}</span>
+            {% if p.hostname %}<br><span class="hostname">{{ p.hostname }}</span>{% endif %}
+          </td>
           <td class="text-muted traffic">
             <span title="Получено">↓ {{ p.rx }}</span>
             <span title="Отправлено">↑ {{ p.tx }}</span>