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/components/PlayView.tsx

@@ -73,6 +73,9 @@ export function PlayView({
             pointerEvents: "none",
             opacity: 0.92,
             animation: "dolphinSwim 62s linear infinite",
+            // Freezes in its current pose rather than snapping back to frame 0 -
+            // `animation: "none"` would restart it, `animationPlayState` just pauses.
+            animationPlayState: state.playing ? "running" : "paused",
             filter: "drop-shadow(0 10px 26px oklch(10% 0.05 210 / .45))",
           }}
         />