Add --version / version with baked-in git commit

ldflags inject version, commit, and build date; a version subcommand
also reports go version and platform. Ends the which-binary-is-this
guessing after deploys.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/rtsp-streamer/main.go

@@ -17,10 +17,18 @@ var (
 	logLevel string
 )
 
+// Build metadata, injected via -ldflags -X at build time (see Makefile).
+var (
+	version = "dev"
+	commit  = "none"
+	date    = "unknown"
+)
+
 func main() {
 	root := &cobra.Command{
 		Use:           "rtsp-streamer",
 		Short:         "Full-screen RTSP camera wall for UniFi Protect",
+		Version:       fmt.Sprintf("%s (commit %s, built %s)", version, commit, date),
 		SilenceUsage:  true,
 		SilenceErrors: true,
 	}
@@ -34,6 +42,7 @@ func main() {
 		statusCmd(),
 		tuiCmd(),
 		configCmd(),
+		versionCmd(),
 	)
 
 	if err := root.Execute(); err != nil {