feat(snmp): surface SNMP device readings on the matching host page

SNMP devices are config-defined by IP/hostname, not Steward Host records, so
they had no presence on a host's page. Map them by address and embed a fragment
(mirrors the Docker per-host fragment).

- _devices_for_host(devices_cfg, address, name): case-insensitive match of a
  device's configured host to the Steward host's address or name (tolerates
  non-dict / host-less entries).
- Route /plugins/snmp/host/<id>: renders the matched device(s) + latest readings,
  or nothing when none map (so hosts without SNMP carry no empty card).
- snmp/host_panel.html: per-device card (name · address · reachability) with a
  readings grid (K/M scaling) and a History link to the full device page.
- hosts/detail.html embeds it after the Docker fragment, gated on snmp enabled.
- Unit test for the matching helper (by address, by name, no-match, blank).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

steward/templates/hosts/detail.html

@@ -224,6 +224,11 @@
 <div hx-get="/plugins/docker/host/{{ host.id }}" hx-trigger="load" hx-swap="innerHTML"></div>
 {% endif %}
 
+{# ── SNMP (renders nothing unless a configured SNMP device maps to this host) ── #}
+{% if "snmp" in enabled_plugins %}
+<div hx-get="/plugins/snmp/host/{{ host.id }}" hx-trigger="load" hx-swap="innerHTML"></div>
+{% endif %}
+
 {{ toggle_script() }}
 <script>var _mt=document.getElementById('mtype'); if(_mt) mtoggle(_mt.value);</script>
 {% endblock %}