server: /metrics endpoint + signaling stub for v0.3 WebRTC

- Add Prometheus counters/gauges/histograms: messages_total{source},
  message_bytes_total, active_subscribers, publish_duration_seconds.
- Add /api/signal/<room> mailbox endpoint (POST adds, GET drains).
  Currently scaffolding for WebRTC SDP/ICE exchange — peers do not
  use it yet; client-side WebRTC negotiation is roadmap.

client: structured default label

Use "<GOOS>-sse-<role>" (e.g., windows-sse-listener) instead of the
old "linux-client" fallback. -label still overrides.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

client/main.go

@@ -12,12 +12,20 @@ import (
 	"log"
 	"net/http"
 	"os"
+	"runtime"
 	"strings"
 	"time"
 
 	"github.com/atotto/clipboard"
 )
 
+// defaultLabel returns "{platform}-{transport}-{role}" so the server can
+// tell at a glance who's connecting. Transport is "sse" today; "rtc" once
+// WebRTC lands (v0.3 roadmap).
+func defaultLabel(role string) string {
+	return fmt.Sprintf("%s-sse-%s", runtime.GOOS, role)
+}
+
 var (
 	noClipboard bool
 	myLabel     string
@@ -31,10 +39,18 @@ type Message struct {
 
 func main() {
 	server := flag.String("server", "https://tether.pecord.io", "tether-server base URL")
-	label := flag.String("label", "linux-client", "X-Tether-Client label")
+	label := flag.String("label", "", "X-Tether-Client label (default: <os>-sse-<role>)")
 	sendText := flag.String("send", "", "send this text and exit (otherwise listen)")
 	flag.BoolVar(&noClipboard, "no-clipboard", false, "don't write incoming messages to the OS clipboard")
 	flag.Parse()
+
+	if *label == "" {
+		if *sendText != "" {
+			*label = defaultLabel("sender")
+		} else {
+			*label = defaultLabel("listener")
+		}
+	}
 	myLabel = *label
 
 	if *sendText != "" {