Add librosa beat/mood analysis and an Ambience background driven by it

The background worker now runs a real librosa analyzer (tempo, beat grid,
per-second energy/valence curves) instead of only the null baseline, kept
behind build_analyzer() so a plain checkout without the analysis extra still
runs fine. The web player reads that per-track analysis and drives a new
animated "Ambience" background (bubbles, colour, current) that reacts to the
beat and the mood curve as the track plays, plus a debug overlay for tuning
it. Also adds a one-off script to backfill podcast cover art from iTunes.

web/src/api/client.ts

@@ -1,6 +1,6 @@
 /** Every call the UI makes. Commands are fire-and-forget: the websocket reports back. */
 
-import type { Album, HaConfig, HaEntityState, PlayerState, Settings } from "./types";
+import type { Album, HaConfig, HaEntityState, PlayerState, Settings, TrackDetail } from "./types";
 
 async function request<T>(path: string, init?: RequestInit): Promise<T> {
   const response = await fetch(`/api${path}`, {
@@ -47,6 +47,14 @@ async function fetchHaStates(entityIds: string[]): Promise<Record<string, HaEnti
   return byId;
 }
 
+/** `null` means "not analyzed" (the backend's expected 404 for this), not an error -
+ * the ambient background just falls back to its un-analyzed baseline for that track. */
+async function fetchTrackDetail(albumId: string, trackIndex: number): Promise<TrackDetail | null> {
+  const response = await fetch(`/api/tracks/${albumId}/${trackIndex}/analysis`);
+  if (!response.ok) return null;
+  return (await response.json()) as TrackDetail;
+}
+
 export const api = {
   library: () => request<{ albums: Album[] }>("/library").then((body) => body.albums),
   state: () => request<PlayerState>("/state"),
@@ -70,6 +78,8 @@ export const api = {
   haStates: fetchHaStates,
   haCallService: (domain: string, service: string, body: Record<string, unknown>) =>
     post(`/ha/services/${domain}/${service}`, body),
+
+  trackDetail: fetchTrackDetail,
 };
 
 export const coverUrl = (albumId: string) => `/api/albums/${albumId}/cover`;