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/globals/globals.go

@@ -0,0 +1,31 @@
+// Package globals provides build-time variables injected via
+// ldflags and made available through dependency injection.
+package globals
+
+import "go.uber.org/fx"
+
+//nolint:gochecknoglobals // set from main before fx starts
+var (
+	// Appname is the application name.
+	Appname string
+	// Version is the git version tag.
+	Version string
+	// Buildarch is the build architecture.
+	Buildarch string
+)
+
+// Globals holds build-time metadata for the application.
+type Globals struct {
+	Appname   string
+	Version   string
+	Buildarch string
+}
+
+// New creates a Globals instance from package-level variables.
+func New(_ fx.Lifecycle) (*Globals, error) {
+	return &Globals{
+		Appname:   Appname,
+		Buildarch: Buildarch,
+		Version:   Version,
+	}, nil
+}