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.

python-backend/musicmouse/services/web/api.py

@@ -34,7 +34,6 @@ from musicmouse.events import (
 from musicmouse.services.web.hub import StateHub
 from musicmouse.services.web.schemas import (
     AlbumOut,
-    BeatsOut,
     HaConfigOut,
     HaDeviceOut,
     LibraryOut,
@@ -43,6 +42,8 @@ from musicmouse.services.web.schemas import (
     SeekIn,
     SettingsIn,
     SettingsOut,
+    TrackCurvesOut,
+    TrackDetailOut,
     VolumeIn,
 )
 from musicmouse.services.web.settings import (
@@ -86,11 +87,16 @@ def build_router(
         )
 
     @router.get("/tracks/{album_id}/{index}/analysis")
-    def get_track_analysis(album_id: str, index: int) -> BeatsOut:
+    def get_track_analysis(album_id: str, index: int) -> TrackDetailOut:
         grid = app.library.beats(album_id, index)
-        if grid is None:
+        curve = app.library.curve(album_id, index)
+        if grid is None and curve is None:
             raise HTTPException(status_code=404, detail="not analyzed")
-        return BeatsOut(times=list(grid.times), strengths=list(grid.strengths))
+        return TrackDetailOut(
+            times=list(grid.times) if grid else [],
+            strengths=list(grid.strengths) if grid else [],
+            curve=TrackCurvesOut(**curve.to_json()) if curve else None,
+        )
 
     @router.post("/library/refresh", status_code=202)
     async def refresh_library() -> Response: