feat: pod-wide Lyria background music (replaces test-audio drums)

The 'Test audio' button becomes 'Background Music': each pod gets a calm looping track from Gemini Lyria 3 that sings the pod name once up front then stays instrumental. Backend GET /api/pods/:id/music generates via the Gemini interactions endpoint and caches the MP3 per pod in Mongo (pod_music); the frontend fetches and loops it via Web Audio, published pod-wide on the existing podman-beat track.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/api.ts

@@ -106,6 +106,11 @@ export function podActivityStreamUrl(id: string): string {
   return `${BACKEND_URL}/api/pods/${encodeURIComponent(id)}/activity/stream`;
 }
 
+/** URL of the pod's generated background-music MP3 (looped client-side). */
+export function podMusicUrl(id: string): string {
+  return `${BACKEND_URL}/api/pods/${encodeURIComponent(id)}/music`;
+}
+
 export async function getMemberWorkHistory(
   podId: string,
   member: string,