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>

backend/src/server.ts

@@ -29,6 +29,7 @@ import { loadPodGraph, reachFrom } from './graph/store.js';
 import { listPodActivity } from './activity/store.js';
 import { getMemberWorkHistory } from './activity/member-history.js';
 import { speakInRoom } from './voice/live.js';
+import { getPodMusic } from './voice/music.js';
 import { notifyHermesInterventionInRoom } from './action/hermes.js';
 import {
   activeLiveConversation,
@@ -406,6 +407,21 @@ app.get('/api/internal/hermes/jobs/:jobId/events/stream', async (req, res) => {
   });
 });
 
+// Per-pod background music (Lyria), generated once and cached. Streams MP3 the
+// frontend loops as a pod-wide LiveKit track (replaces the synthesized beat).
+app.get('/api/pods/:id/music', async (req, res) => {
+  try {
+    const pod = await getPod(req.params.id);
+    if (!pod) return res.status(404).json({ error: 'pod not found' });
+    const mp3 = await getPodMusic(pod.id, pod.name);
+    res.set('Content-Type', 'audio/mpeg');
+    res.set('Cache-Control', 'public, max-age=86400');
+    res.send(mp3);
+  } catch (e) {
+    res.status(500).json({ error: (e as Error).message });
+  }
+});
+
 app.post('/api/pods/:id/hermes/notify', async (req, res) => {
   const podId = req.params.id;
   const pod = await getPod(podId);

backend/src/voice/music.ts

@@ -0,0 +1,94 @@
+import { Buffer } from 'node:buffer';
+import { getDb } from '../memory/db.js';
+import { env } from '../env.js';
+
+// Lyria 3 is reached via the Gemini "interactions" endpoint (not :predict, which
+// is the Vertex path). The clip model returns a ~30s base64 MP3.
+const MUSIC_MODEL = process.env.GEMINI_MUSIC_MODEL ?? 'lyria-3-clip-preview';
+const INTERACTIONS_URL = 'https://generativelanguage.googleapis.com/v1beta/interactions';
+
+interface PodMusicDoc {
+  podId: string;
+  name: string; // pod name the vocal was generated for
+  model: string;
+  mp3Base64: string;
+  createdAt: string;
+}
+
+interface InteractionContent {
+  type?: string;
+  data?: string;
+  text?: string;
+}
+interface InteractionResponse {
+  steps?: Array<{ content?: InteractionContent[] }>;
+  output_audio?: { data?: string };
+}
+
+/**
+ * Background "hold music" prompt: opens with the pod name sung once, then a calm
+ * instrumental bed that loops. Keep it unobtrusive — this is fill, not a song.
+ */
+function musicPrompt(podName: string): string {
+  return [
+    'Calm soothing instrumental background hold music for a tech app, like gentle on-hold lobby music.',
+    `It opens in the first three seconds with a soft gentle voice clearly saying the words "${podName}" one time,`,
+    'and after that opening it is purely instrumental with warm electric piano, gentle synth pads and a soft relaxed beat.',
+    'Unobtrusive, pleasant and steady with no climax, designed to loop seamlessly as quiet background fill.',
+    'No other lyrics or vocals after the opening.',
+  ].join(' ');
+}
+
+function extractAudioBase64(data: InteractionResponse): string | null {
+  for (const step of data.steps ?? []) {
+    for (const c of step.content ?? []) {
+      if (c.type === 'audio' && c.data) return c.data;
+    }
+  }
+  return data.output_audio?.data ?? null;
+}
+
+async function generate(podName: string): Promise<Buffer> {
+  const res = await fetch(INTERACTIONS_URL, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json', 'x-goog-api-key': env.GEMINI_API_KEY },
+    body: JSON.stringify({ model: MUSIC_MODEL, input: musicPrompt(podName) }),
+  });
+  if (!res.ok) {
+    throw new Error(`Lyria ${res.status}: ${(await res.text()).slice(0, 300)}`);
+  }
+  const data = (await res.json()) as InteractionResponse;
+  const b64 = extractAudioBase64(data);
+  if (!b64) throw new Error('Lyria returned no audio');
+  return Buffer.from(b64, 'base64');
+}
+
+/**
+ * The pod's background-music MP3, generated by Lyria on first request and cached
+ * in the `pod_music` collection. Regenerated if the pod name changes so the sung
+ * name stays correct. Lyria generation is slow (~20s); the cache makes every
+ * call after the first instant.
+ */
+export async function getPodMusic(podId: string, podName: string): Promise<Buffer> {
+  const db = await getDb();
+  const col = db.collection<PodMusicDoc>('pod_music');
+  const cached = await col.findOne({ podId });
+  if (cached && cached.name === podName && cached.model === MUSIC_MODEL && cached.mp3Base64) {
+    return Buffer.from(cached.mp3Base64, 'base64');
+  }
+  const mp3 = await generate(podName);
+  await col.updateOne(
+    { podId },
+    {
+      $set: {
+        podId,
+        name: podName,
+        model: MUSIC_MODEL,
+        mp3Base64: mp3.toString('base64'),
+        createdAt: new Date().toISOString(),
+      },
+    },
+    { upsert: true },
+  );
+  return mp3;
+}