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/hooks/useTrackDetail.ts

@@ -0,0 +1,44 @@
+/** The current track's beat grid and mood/drive curves, fetched lazily - never inside
+ * the library payload, and never for a track that has no analysis of any kind.
+ *
+ * Fetched whenever the current track is music and has *any* analysis - unlike the
+ * beat grid's own trustworthiness gate (a spoken-word track or a free-tempo lullaby
+ * always produces *some* grid from the beat tracker, and pulsing the background to a
+ * beat nobody can actually hear looks broken), which is a decision about whether the
+ * already-fetched beats are used, not whether they're fetched at all - see the
+ * `PULSE_THRESHOLD` gate in `Ambience.tsx`. The energy/valence/drive curves this also
+ * carries have no such caveat: they drive color and current for every analyzed music
+ * track regardless of beat quality.
+ */
+
+import { useEffect, useState } from "react";
+import { api } from "../api/client";
+import type { Album, PlayerState, TrackDetail } from "../api/types";
+import { groupOf } from "../lib/search";
+
+export function useTrackDetail(album: Album | null, state: PlayerState): TrackDetail | null {
+  const [detail, setDetail] = useState<TrackDetail | null>(null);
+
+  const albumId = state.album_id;
+  const trackIndex = state.track_index;
+  const analysis = album?.tracks[trackIndex]?.analysis ?? null;
+  const eligible =
+    album !== null && albumId !== null && groupOf(album) === "music" && analysis !== null;
+
+  useEffect(() => {
+    // Cleared up front, not just on failure - a track change must not keep the
+    // previous track's detail visible while the new one is still in flight.
+    setDetail(null);
+    if (!eligible || albumId === null) return;
+
+    let cancelled = false;
+    void api.trackDetail(albumId, trackIndex).then((result) => {
+      if (!cancelled) setDetail(result);
+    });
+    return () => {
+      cancelled = true;
+    };
+  }, [eligible, albumId, trackIndex]);
+
+  return detail;
+}