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

tests/plugins/snmp/test_host_match.py

@@ -0,0 +1,29 @@
+"""Unit tests for mapping config-defined SNMP devices onto a Steward host."""
+from plugins.snmp.routes import _devices_for_host
+
+_CFG = [
+    {"name": "core-switch", "host": "192.168.1.1"},
+    {"name": "ups", "host": "192.168.1.10"},
+    {"name": "by-name", "host": "edge-router"},
+    "not-a-dict",                     # tolerated, skipped
+    {"name": "no-host"},              # no host → never matches
+]
+
+
+def test_matches_by_address():
+    out = _devices_for_host(_CFG, "192.168.1.1", "somehost")
+    assert [d["name"] for d in out] == ["core-switch"]
+
+
+def test_matches_by_name_case_insensitive():
+    out = _devices_for_host(_CFG, "10.0.0.9", "Edge-Router")
+    assert [d["name"] for d in out] == ["by-name"]
+
+
+def test_no_match_returns_empty():
+    assert _devices_for_host(_CFG, "10.0.0.1", "nope") == []
+
+
+def test_blank_address_and_name_match_nothing():
+    # A host with no address/name must not match a device with a blank host.
+    assert _devices_for_host(_CFG, "", None) == []