Add .gitea/workflows/check.yml that builds both the root and
backend Docker images on push. Add LICENSE and README.md to the
backend subproject to match repo standards.
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.
