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/internal/handlers/healthcheck.go

@@ -0,0 +1,11 @@
+package handlers
+
+import "net/http"
+
+// HandleHealthCheck returns a handler for the health check
+// endpoint.
+func (s *Handlers) HandleHealthCheck() http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		s.respondJSON(w, r, s.hc.Check(), http.StatusOK)
+	}
+}