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

plugins/snmp/templates/snmp/host_panel.html

@@ -0,0 +1,47 @@
+{# Per-host SNMP fragment — embedded on the Hosts hub via HTMX. Self-contained.
+   Shows the SNMP device(s) whose address maps to this host + latest readings. #}
+<div class="card">
+  <div style="display:flex;align-items:baseline;justify-content:space-between;margin-bottom:0.6rem;gap:1rem;flex-wrap:wrap;">
+    <h3 class="section-title" style="margin-bottom:0;">SNMP</h3>
+    <a href="/plugins/snmp/" style="font-size:0.8rem;color:var(--text-muted);">All devices →</a>
+  </div>
+
+  {% for device in devices %}
+  <div style="{% if not loop.last %}margin-bottom:1rem;{% endif %}">
+    <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.readings %}
+      <span style="font-size:0.74rem;color:var(--green);">&#9679; reachable</span>
+      {% else %}
+      <span style="font-size:0.74rem;color:var(--text-dim);">&#9675; no data yet</span>
+      {% endif %}
+      <a href="/plugins/snmp/device/{{ device.name }}" class="btn btn-ghost btn-sm"
+         style="margin-left:auto;font-size:0.76rem;padding:0.15rem 0.55rem;">History</a>
+    </div>
+
+    {% if device.readings %}
+    <div style="display:grid;grid-template-columns:repeat(auto-fill,minmax(150px,1fr));gap:0.5rem;">
+      {% for oid_cfg in device.oids %}
+      {% set label = oid_cfg.label if oid_cfg.label else oid_cfg.oid %}
+      {% set val = device.readings.get(label) %}
+      <div style="background:var(--bg-elevated);border-radius:5px;padding:0.5rem 0.7rem;border:1px solid var(--border);">
+        <div style="font-size:0.68rem;color:var(--text-dim);margin-bottom:0.2rem;font-family:ui-monospace,monospace;white-space:nowrap;overflow:hidden;text-overflow:ellipsis;">{{ label }}</div>
+        {% if val is not none %}
+        <div style="font-size:1rem;font-weight:600;font-variant-numeric:tabular-nums;">
+          {% if val >= 1000000 %}{{ "%.2f"|format(val / 1000000) }}<span style="font-size:0.72rem;color:var(--text-muted);">M</span>
+          {% elif val >= 1000 %}{{ "%.1f"|format(val / 1000) }}<span style="font-size:0.72rem;color:var(--text-muted);">K</span>
+          {% else %}{{ "%.4g"|format(val) }}{% endif %}
+        </div>
+        {% else %}
+        <div style="font-size:0.85rem;color:var(--text-dim);">—</div>
+        {% endif %}
+      </div>
+      {% endfor %}
+    </div>
+    {% else %}
+    <p style="font-size:0.8rem;color:var(--text-dim);margin:0;">No readings yet — waiting for the next poll.</p>
+    {% endif %}
+  </div>
+  {% endfor %}
+</div>