feat(traffic): a host's top senders, on its page and as get_host_senders (#6115)

The host page gains a "Who's sending" panel, and the MCP a tool, that
put the host's outbound (or inbound) traffic down to its containers and
NFS mounts: read in shared time buckets, so the host's busiest bucket
says who was sending in it. A swarm service's tasks add up under its
name; what none of them accounts for is the host itself.

Also fixes CI 9488: per-container net_rx_bps / net_tx_bps join the
docker alert catalog, so a rule can catch a container flooding the
uplink.

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

plugins/host_agent/templates/host_detail.html

@@ -42,6 +42,14 @@
   </div>
   {% endif %}
 </div>
+<div id="hm-senders" class="card" style="margin-top:1rem;margin-bottom:0;"
+     hx-get="/plugins/host_agent/{{ host.id }}/senders"
+     hx-trigger="load, every {{ chart_poll_seconds }}s, rangeChange from:body"
+     hx-include="#time-range"
+     hx-swap="innerHTML">
+  <div class="section-title" style="margin-bottom:0.5rem;">Who's sending</div>
+  <p class="field-hint" style="margin:0;">Loading…</p>
+</div>
 {% if "syslog" in enabled_plugins %}<p id="hm-marks-note" class="field-hint" hidden style="margin:0.5rem 0 0;"></p>{% endif %}
 {# Hidden poller: fetches the history series and applies it to the charts above
    (no visible swap). Lazy on load → never blocks paint; refreshed slowly + on