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:

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

@@ -15,7 +15,6 @@ from musicmouse.library.analysis import TrackAnalysis
 
 __all__ = [
     "AlbumOut",
-    "BeatsOut",
     "HaConfigOut",
     "HaDeviceOut",
     "LibraryOut",
@@ -24,6 +23,8 @@ __all__ = [
     "SeekIn",
     "SettingsIn",
     "SettingsOut",
+    "TrackCurvesOut",
+    "TrackDetailOut",
     "TrackOut",
     "VolumeIn",
 ]
@@ -34,6 +35,8 @@ class AnalysisOut(BaseModel):
     energy: float | None = None
     valence: float | None = None
     brightness: float | None = None
+    #: 0..1 confidence that `tempo` is an audible, steady beat - see `TrackAnalysis`.
+    pulse: float | None = None
     beats: bool = False
 
     @classmethod
@@ -45,6 +48,7 @@ class AnalysisOut(BaseModel):
             energy=analysis.energy,
             valence=analysis.valence,
             brightness=analysis.brightness,
+            pulse=analysis.pulse,
             beats=analysis.beats,
         )
 
@@ -99,9 +103,23 @@ class LibraryOut(BaseModel):
     albums: list[AlbumOut]
 
 
-class BeatsOut(BaseModel):
+class TrackCurvesOut(BaseModel):
+    hop_seconds: float
+    energy: list[float]
+    valence: list[float]
+    drive: list[float]
+
+
+class TrackDetailOut(BaseModel):
+    """Beats + mood/drive curves for the one track currently playing - fetched
+    together since both are per-track detail the library payload never carries.
+    ``curve`` is ``None`` only when the whole track failed analysis; ``times``/
+    ``strengths`` are empty (not ``None``) for a track with no reliable beat, since a
+    free-tempo or spoken-word track still has real energy/valence/drive curves."""
+
     times: list[float]
     strengths: list[float]
+    curve: TrackCurvesOut | None = None
 
 
 class ConnectionOut(BaseModel):