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>

Makefile

@@ -0,0 +1,12 @@
+BIN := bin/energy-mqtt-sub
+
+.PHONY: build run clean
+
+build:
+	go build -o $(BIN) .
+
+run: build
+	./$(BIN) -c ./config/config.json
+
+clean:
+	rm -f $(BIN)