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/reportbuf/reportbuf_test.go

@@ -0,0 +1,13 @@
+package reportbuf_test
+
+import (
+	"testing"
+
+	_ "sneak.berlin/go/netwatch/internal/reportbuf"
+)
+
+func TestImport(t *testing.T) {
+	t.Parallel()
+	// Compilation check — verifies the package parses
+	// and all imports resolve.
+}