Resolve host IPs through DNS, add purge, French date format by default

- Reverse DNS (cached PTR lookups): IP hosts are stored with their name
  in 'host' and the IP in 'host_ip'; older IP-only logs are resolved on
  display and the host filter shows 'name (IP)'. RDNS / DNS_SERVER env.
- Settings > Danger zone: delete all logs (type PURGE to confirm) through
  VictoriaLogs /delete/run_task; -delete.enable added to docker-compose.
  ALLOW_PURGE env to disable it.
- Remove the custom YYYY-MM-DD - HH:MM:SS:mmm format; default is now the
  usual French display DD/MM/YYYY HH:MM:SS.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/index.html

@@ -125,6 +125,15 @@
           <button id="resetTags" class="btn ghost" type="button" data-i18n="resetTags">Restore default tags</button>
         </footer>
       </section>
+
+      <section class="set-section danger">
+        <h3 data-i18n="dangerZone">Danger zone</h3>
+        <p class="muted small" data-i18n="purgeHelp"></p>
+        <div class="purge-row">
+          <button id="purgeBtn" class="btn danger" type="button" data-i18n="purgeBtn">Delete all logs…</button>
+          <span id="purgeStatus" class="purge-status muted" hidden></span>
+        </div>
+      </section>
     </div>
   </dialog>