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/lib/ambienceTunables.ts

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+/** The 5 parameters `?debugDynamicUI=1` can adjust live, session-only (plain React
+ * state in App.tsx, no persistence). Every other constant this refactor introduces
+ * lives in `lib/ambience.ts` / `components/Ambience.tsx` as a plain named constant -
+ * that pair of files plus this one's `DEFAULT_TUNABLES` is the complete answer to
+ * "what do I edit to make a debug-slider tweak permanent."
+ */
+
+export interface AmbienceTunables {
+  /** Multiplies energy's swing on chroma/lightness. 1.0 = default. */
+  energyColorGain: number;
+  /** Multiplies how much a beat crossing boosts current magnitude, on top of the
+   * always-present tempo x drive-derived base. 0 = no effect. */
+  beatCurrentFactor: number;
+  /** Multiplies the sampled `drive` curve before it modulates current magnitude (see
+   * `currentMagnitudeAt`). 1.0 = default; 0 = current ignores drive entirely and sits
+   * at `DRIVE_MAGNITUDE_FLOOR` x base magnitude regardless of the music. */
+  driveCurrentGain: number;
+  /** How much bigger a bubble gets by the time it's risen a full screen height, e.g.
+   * 0.4 = 40% larger at the top. */
+  bubbleGrowthRate: number;
+  /** Seconds: exponential smoothing time constant for how quickly the curve-sampled
+   * energy/valence/drive feeding colour and current track the underlying (already
+   * linearly-interpolated) curve - a frontend preview knob, distinct from the
+   * backend's fixed analysis-time sampling rate (`_ANALYSIS_HOP_SECONDS` in
+   * `librosa_analyzer.py`, which needs a re-analysis to change). 0 = no extra
+   * smoothing. */
+  curveSmoothingTau: number;
+}
+
+export const DEFAULT_TUNABLES: AmbienceTunables = {
+  energyColorGain: 1.0,
+  beatCurrentFactor: 0.6,
+  driveCurrentGain: 1.0,
+  bubbleGrowthRate: 0.4,
+  curveSmoothingTau: 0.4,
+};
+
+/** Drives the whole visual pipeline from these values instead of real playback data -
+ * a "playground mode" for developing a feel for the effect without needing music
+ * playing. Only takes effect while nothing is actually playing (`!state.playing`) -
+ * real playback always wins the instant it starts, so there's never a fight between
+ * the two. Session-only, same as `AmbienceTunables`; distinct from it conceptually:
+ * tunables are *parameters* (how strongly the system reacts), this is *input*
+ * (pretending to be the music). */
+export interface ManualControl {
+  enabled: boolean;
+  energy: number; // 0..1
+  valence: number; // 0..1
+  tempo: number; // bpm - also stands in for the current's base magnitude and rise
+  /** 0..1 - stands in for the sampled `drive` curve, since there's no real track to
+   * derive rhythmic intensity from. 1.0 default = full current magnitude, matching
+   * the flat-curve fallback's own default (see `effectiveCurve`). */
+  drive: number;
+  /** Strength of each synthetic beat pulse, auto-generated at the `tempo` above while
+   * manual mode is active - a continuous fake metronome rather than a one-shot
+   * trigger button, so the beat-driven effects (spawn burst + current kick) can be
+   * watched continuously rather than poked one click at a time. */
+  beatStrength: number; // 0..1
+}
+
+export const DEFAULT_MANUAL_CONTROL: ManualControl = {
+  enabled: false,
+  energy: 0.5,
+  valence: 0.5,
+  tempo: 120,
+  drive: 1.0,
+  beatStrength: 0.8,
+};