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/styles/app.css

@@ -87,6 +87,9 @@ button {
   );
 }
 
+/* Still used by the room-control page's own `<Bubbles>` field (`RoomView.tsx`) - the
+   main player's bubbles moved to `Ambience.tsx`'s canvas, which needs real spawn and
+   turbulence control that a fixed set of CSS animations cannot give it. */
 .bubble {
   position: absolute;
   bottom: -40px;
@@ -97,6 +100,20 @@ button {
   pointer-events: none;
 }
 
+.ambience {
+  display: block;
+  /* A canvas is a replaced element: without an explicit size here, its CSS layout box
+     falls back to its own width/height *attributes* - the very thing the resize
+     handler sets from this box's measured size times devicePixelRatio. On any display
+     where dpr != 1, that is a feedback loop: each resize multiplies the attribute
+     (and thus, without this rule, the box) by dpr again, diverging within a few
+     frames until the canvas exceeds the browser's max size and throws. Pinning the
+     box to 100% of `.stage` breaks the loop - layout no longer depends on the
+     attribute at all. */
+  width: 100%;
+  height: 100%;
+}
+
 .pill {
   border: none;
   cursor: pointer;