feat(redis): implement Pub/Sub support for usage tracking

- Added Redis Pub/Sub capability to broadcast usage updates to subscribed clients.
- Enhanced `redisqueue` with subscriber management and message broadcasting.
- Updated tests to validate Pub/Sub message handling, subscription behavior, and fallback to the queue after unsubscribing.
- Integrated `project_id` parsing into auth-files logic to include project identifiers in metadata.

internal/redisqueue/queue_test.go

@@ -0,0 +1,67 @@
+package redisqueue
+
+import (
+	"testing"
+	"time"
+)
+
+func TestEnqueueBroadcastsToUsageSubscribersAndSkipsQueue(t *testing.T) {
+	withEnabledQueue(t, func() {
+		first, unsubscribeFirst := SubscribeUsage()
+		defer unsubscribeFirst()
+		second, unsubscribeSecond := SubscribeUsage()
+		defer unsubscribeSecond()
+
+		Enqueue([]byte("usage-record"))
+
+		requireUsageSubscriberPayload(t, first, "usage-record")
+		requireUsageSubscriberPayload(t, second, "usage-record")
+
+		if items := PopOldest(1); len(items) != 0 {
+			t.Fatalf("PopOldest() items = %q, want empty after subscriber broadcast", items)
+		}
+
+		unsubscribeFirst()
+		unsubscribeSecond()
+
+		Enqueue([]byte("queued-record"))
+		items := PopOldest(1)
+		if len(items) != 1 || string(items[0]) != "queued-record" {
+			t.Fatalf("PopOldest() items = %q, want queued record after unsubscribe", items)
+		}
+	})
+}
+
+func TestSetEnabledFalseClosesUsageSubscribers(t *testing.T) {
+	withEnabledQueue(t, func() {
+		subscriber, unsubscribe := SubscribeUsage()
+		defer unsubscribe()
+
+		SetEnabled(false)
+
+		select {
+		case _, ok := <-subscriber:
+			if ok {
+				t.Fatalf("subscriber channel remained open after SetEnabled(false)")
+			}
+		case <-time.After(time.Second):
+			t.Fatalf("timeout waiting for subscriber close")
+		}
+	})
+}
+
+func requireUsageSubscriberPayload(t *testing.T, subscriber <-chan []byte, want string) {
+	t.Helper()
+
+	select {
+	case got, ok := <-subscriber:
+		if !ok {
+			t.Fatalf("subscriber closed before receiving %q", want)
+		}
+		if string(got) != want {
+			t.Fatalf("subscriber payload = %q, want %q", string(got), want)
+		}
+	case <-time.After(time.Second):
+		t.Fatalf("timeout waiting for subscriber payload %q", want)
+	}
+}