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>

Dockerfile

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+FROM golang:1.24-alpine AS builder
+WORKDIR /app
+COPY go.mod go.sum ./
+RUN go mod download
+COPY *.go ./
+RUN CGO_ENABLED=0 go build -o energy-mqtt-sub .
+
+FROM alpine:3.21
+RUN apk add --no-cache ca-certificates tzdata
+WORKDIR /app
+COPY --from=builder /app/energy-mqtt-sub .
+USER nobody:nobody
+ENTRYPOINT ["./energy-mqtt-sub", "-c", "/srv/energy/meter/config.json"]