feat(snmp): explicit host binding (host_id / steward_host) with implicit fallback

SNMP devices map onto a Steward host's detail page via _devices_for_host,
which previously matched the device's `host` field (the SNMP poll target)
against the Host's address/name — a coincidence of strings that breaks when
the poll target differs from how the Host is recorded.

Add two optional per-device config fields that decouple the binding from the
poll target:
  • host_id      — exact Steward Host UUID match (the explicit link)
  • steward_host — friendly bind by Host name or address (case-insensitive)

An explicit binding is exclusive: a device bound to host A never implicitly
matches host B by a coincidental address. No explicit binding → existing
implicit `host`-string match (fully backward compatible).

host_panel passes host.id and badges explicitly-bound devices. plugin.yaml
documents the new fields. Tests cover explicit id/name, exclusivity, wrong
uuid, and legacy fallback.

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

plugins/snmp/templates/snmp/host_panel.html

@@ -11,6 +11,10 @@
     <div style="display:flex;align-items:center;gap:0.6rem;margin-bottom:0.5rem;flex-wrap:wrap;">
       <span style="font-weight:600;font-size:0.92rem;">{{ device.name }}</span>
       <span style="font-size:0.78rem;color:var(--text-dim);font-family:ui-monospace,monospace;">{{ device.host }}</span>
+      {% if device.bound %}
+      <span title="Explicitly bound to this host (host_id / steward_host)"
+            style="font-size:0.68rem;color:var(--text-muted);background:var(--bg-elevated);border:1px solid var(--border);border-radius:3px;padding:0.05rem 0.4rem;">bound</span>
+      {% endif %}
       {% if device.readings %}
       <span style="font-size:0.74rem;color:var(--green);">&#9679; reachable</span>
       {% else %}