Update base image from golang:1.24-alpine to golang:1.25-alpine
to match go.mod requirement. Install golangci-lint by pinned commit
hash so make check passes inside the container. Update runtime
image to alpine:3.23.
Introduce the Go backend (netwatch-server) with an HTTP API that
accepts telemetry reports and persists them as zstd-compressed JSONL
files. Reports are buffered in memory and flushed to disk when the
buffer reaches 10 MiB or every 60 seconds.
