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/__main__.py

@@ -17,7 +17,7 @@ import asyncio
 import contextlib
 import logging
 import sys
-from collections.abc import Coroutine
+from collections.abc import Awaitable, Callable, Coroutine
 from pathlib import Path
 from typing import Any
 
@@ -31,6 +31,7 @@ from musicmouse.devices.null_transport import NullTransport
 from musicmouse.devices.player import Player, VlcPlayer
 from musicmouse.devices.serial_link import SerialLink
 from musicmouse.library import MusicLibrary
+from musicmouse.library.analysis import build_analyzer
 from musicmouse.reactions import register_all
 from musicmouse.services.base import Service
 from musicmouse.services.mqtt import MqttService, build_entities
@@ -43,9 +44,7 @@ def parse_args(argv: list[str] | None = None) -> argparse.Namespace:
     parser = argparse.ArgumentParser(
         prog="musicmouse", description="Host backend for the MusicMouse RFID music player."
     )
-    parser.add_argument(
-        "-c", "--config", type=Path, required=True, help="path to config.yml"
-    )
+    parser.add_argument("-c", "--config", type=Path, required=True, help="path to config.yml")
     parser.add_argument(
         "-s",
         "--simulate",
@@ -127,9 +126,7 @@ async def run_real(config: Config, config_path: Path, *, hardware: bool = True)
             clock=clock,
         )
         mouse = MusicMouseDevice(bus, link, config.tag_map, port=general.serial_port)
-        link.attach(
-            mouse.feed, on_connect=mouse.on_connected, on_disconnect=mouse.on_disconnected
-        )
+        link.attach(mouse.feed, on_connect=mouse.on_connected, on_disconnect=mouse.on_disconnected)
     else:
         mouse = MusicMouseDevice(bus, NullTransport(), config.tag_map, port="none")
 
@@ -137,9 +134,7 @@ async def run_real(config: Config, config_path: Path, *, hardware: bool = True)
 
     library = await build_library(config)
     app = _build_app(config, bus, mouse, player, library, clock=clock)
-    services = _build_services(
-        app, mouse, player, clock=clock, config_path=config_path
-    )
+    services = _build_services(app, mouse, player, clock=clock, config_path=config_path)
 
     _log.info(
         "MusicMouse %s starting: %d figures, %d albums, serial %s, audio %s, mqtt %s",
@@ -157,6 +152,15 @@ async def run_real(config: Config, config_path: Path, *, hardware: bool = True)
             [
                 *([link.run()] if link else []),
                 player.run(),
+                # `is_busy` reads `player.is_playing` directly rather than the library
+                # knowing about playback at all: analysis must never compete with audio
+                # decoding for CPU, and this device is otherwise idle most of the day, so
+                # the pass simply resumes once it is. `on_batch` is unset unless the web
+                # front-end is on - nothing else has a use for the notification.
+                library.run_analysis(
+                    is_busy=lambda: player.is_playing,
+                    on_batch=_analysis_batch_hook(services),
+                ),
                 *(service.run() for service in services),
             ]
         )
@@ -184,6 +188,15 @@ async def run_simulated(config: Config, config_path: Path, script: Path | None)
         sim.app, sim.app.mouse, sim.player, clock=RealClock(), config_path=config_path
     )
     tasks = [asyncio.create_task(service.run(), name=service.name) for service in services]
+    tasks.append(
+        asyncio.create_task(
+            sim.app.library.run_analysis(
+                is_busy=lambda: sim.player.is_playing,
+                on_batch=_analysis_batch_hook(services),
+            ),
+            name="library-analysis",
+        )
+    )
     try:
         if script is not None:
             await run_script_file(sim, script)
@@ -250,6 +263,7 @@ async def build_library(config: Config) -> MusicLibrary:
         library_config.root,
         library_config.cache,
         frozenset(config.general.audio_extensions),
+        analyzer=build_analyzer(),
         figure_kinds=config.figure_kinds,
     )
 
@@ -279,6 +293,16 @@ def _build_app(
     return app
 
 
+def _analysis_batch_hook(services: list[Service]) -> Callable[[], Awaitable[None]] | None:
+    """The web front-end's own hub, if it is running - so open tabs refetch the library
+    as background analysis lands, instead of only after a manual reload. `None` when
+    there is no web service, which `MusicLibrary.analyze_pending` treats as "nobody to
+    tell".
+    """
+    web_service = next((s for s in services if isinstance(s, WebService)), None)
+    return web_service.hub.broadcast_library if web_service else None
+
+
 def _build_services(
     app: App,
     mouse: MusicMouseDevice,