Add SNMP scanner that polls a gateway's ARP table

Introduce a sixth scanner that periodically queries an SNMP agent
(typically the router/firewall) for its ARP/neighbour cache via
SNMPv2c and feeds discovered devices into the shared devices, events
and notifications pipeline. Unlike the ARP scanner it generates no
traffic on the local segment and can surface devices across all
subnets the agent routes.

Scope is intentionally minimal: SNMPv2c only, a single target, and the
ipNetToMediaTable (ARP) only. SNMPv3 and switch MAC/forwarding-table
polling are left as follow-ups in TODO.md.

- backend: csnmp dependency; SnmpScanner config (opt-in, off unless a
  [snmp_scanner] section is present); DeviceEventScanner::Snmp;
  scanners/snmp/{finder,scanner,status}; main.rs wiring; status API
  endpoint wired into OpenAPI
- frontend: SNMP scanner status model, card, API method, status screen
  and summary card rows, and device-event label
- docs/config: sample TOML, README options, NixOS module option, and
  setup notes for enabling SNMP on pfSense/OPNsense

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/model/device_event.dart

@@ -34,6 +34,7 @@ class DeviceEvent {
     'Mdns' => 'mDNS',
     'Ssdp' => 'SSDP/UPnP',
     'Dhcp' => 'DHCP',
+    'Snmp' => 'SNMP',
     _ => scanner,
   };
 }