Add backend with buffered zstd-compressed report storage

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.

backend/.gitignore

@@ -0,0 +1,6 @@
+netwatch-server
+*.log
+*.out
+*.test
+.env
+data/