Initial implementation: MQTT-based energy meter subscriber

Subscribes to Eastron SDM630 power meter data via MQTT broker,
decodes Modbus RTU frames, and writes readings to InfluxDB.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

go.mod

@@ -0,0 +1,15 @@
+module energy-mqtt-sub
+
+go 1.24.5
+
+require (
+	github.com/apapsch/go-jsonmerge/v2 v2.0.0 // indirect
+	github.com/eclipse/paho.mqtt.golang v1.5.1 // indirect
+	github.com/google/uuid v1.3.1 // indirect
+	github.com/gorilla/websocket v1.5.3 // indirect
+	github.com/influxdata/influxdb-client-go/v2 v2.14.0 // indirect
+	github.com/influxdata/line-protocol v0.0.0-20200327222509-2487e7298839 // indirect
+	github.com/oapi-codegen/runtime v1.0.0 // indirect
+	golang.org/x/net v0.44.0 // indirect
+	golang.org/x/sync v0.17.0 // indirect
+)