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/App.tsx

@@ -11,9 +11,10 @@ import { useCallback, useEffect, useMemo, useRef, useState } from "react";
 import { api } from "./api/client";
 import type { Album, HaConfig } from "./api/types";
 import { AlbumModal } from "./components/AlbumModal";
+import { Ambience, type AmbienceDebugSnapshot } from "./components/Ambience";
+import { AmbienceDebugOverlay } from "./components/AmbienceDebugOverlay";
 import { AppHeader } from "./components/AppHeader";
 import { BrowseView } from "./components/BrowseView";
-import { Bubbles } from "./components/Bubbles";
 import { HelpOverlay } from "./components/HelpOverlay";
 import { ParentPanel } from "./components/ParentPanel";
 import { PlayerBar } from "./components/PlayerBar";
@@ -22,6 +23,8 @@ import { RoomView } from "./components/RoomView";
 import { useGridColumns } from "./hooks/useGridColumns";
 import { useLibrary } from "./hooks/useLibrary";
 import { usePlayerState } from "./hooks/usePlayerState";
+import { DEFAULT_MANUAL_CONTROL, DEFAULT_TUNABLES } from "./lib/ambienceTunables";
+import type { AmbienceTunables, ManualControl } from "./lib/ambienceTunables";
 import type { Action, UiState } from "./lib/keyboard";
 import { handleKey, initialUiState } from "./lib/keyboard";
 import { playPop } from "./lib/pop";
@@ -43,6 +46,15 @@ export function App() {
   const [parentMode, setParentMode] = useState(
     () => new URLSearchParams(location.search).get("parentMode") === "1",
   );
+  const [debugDynamicUI] = useState(
+    () => new URLSearchParams(location.search).get("debugDynamicUI") === "1",
+  );
+  const [debugSnapshot, setDebugSnapshot] = useState<AmbienceDebugSnapshot | null>(null);
+  // Exists regardless of `debugDynamicUI` (so `Ambience` always has consistent,
+  // harmless defaults - `manual.enabled` defaults to false) - only the overlay, and
+  // thus the ability to change either, is gated behind the query param.
+  const [tunables, setTunables] = useState<AmbienceTunables>(DEFAULT_TUNABLES);
+  const [manual, setManual] = useState<ManualControl>(DEFAULT_MANUAL_CONTROL);
   // undefined: not yet resolved (hide the nav pill to avoid a flash). null: confirmed
   // absent - the room page is a separate opt-in feature, off by default.
   const [haConfig, setHaConfig] = useState<HaConfig | null | undefined>(undefined);
@@ -228,7 +240,15 @@ export function App() {
 
   return (
     <div className="stage">
-      <Bubbles />
+      {state && (
+        <Ambience
+          album={currentAlbum}
+          state={state}
+          tunables={tunables}
+          manual={manual}
+          onDebugFrame={debugDynamicUI ? setDebugSnapshot : undefined}
+        />
+      )}
 
       {ui.view === "browse" && state && (
         <div
@@ -346,6 +366,17 @@ export function App() {
       {parentMode && <ParentPanel onClose={() => setParentMode(false)} />}
 
       {(!state || library.loading) && <Splash error={library.error} />}
+
+      {debugDynamicUI && debugSnapshot && (
+        <AmbienceDebugOverlay
+          snapshot={debugSnapshot}
+          tunables={tunables}
+          onTunablesChange={setTunables}
+          manual={manual}
+          onManualChange={setManual}
+          playing={state?.playing ?? false}
+        />
+      )}
     </div>
   );
 }