Add optional temperature support to SNMP checker

- New temperature_oid config parameter
- Temperature published as integer via MQTT
- Auto-creates Home Assistant temperature sensor via MQTT Discovery

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

config.yaml.example

@@ -49,6 +49,7 @@ checks:
     host: "192.168.1.2"
     community: "public"
     oid: "1.3.6.1.2.1.1.1.0"  # sysDescr
+    temperature_oid: "1.3.6.1.4.1.14988.1.1.3.10.0"  # Optional, OID varies by vendor
     interval: 120
 
   # DNS checks