feat: library scan — probe seam + scan_library (have + monitored + followed)

Adds the AudioProbe seam (probe.py, _probe.py MutagenProbe with a lazy
mutagen import, FakeAudioProbe) and scan_library, which walks /music,
resolves each album against MusicBrainz (falling back to embedded tags
when the folder name doesn't resolve), and idempotently records matched
albums as have (LibraryItem), monitored (MonitoredRelease, fulfilled),
and followed (WatchedArtist, autoMonitorFuture=false).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

worker/lyra_worker/registry.py

@@ -1,6 +1,7 @@
 from lyra_worker._mbbrowser import MusicBrainzBrowser
 from lyra_worker._musicbrainz import MusicBrainzResolver
 from lyra_worker._mutagen import MutagenTagger
+from lyra_worker._probe import MutagenProbe
 from lyra_worker.adapters._slskd import SlskdClient
 from lyra_worker.adapters._streamrip import StreamripClient
 from lyra_worker.adapters._ytdlp import YtDlpClient
@@ -9,6 +10,7 @@ from lyra_worker.adapters.qobuz import QobuzAdapter
 from lyra_worker.adapters.soulseek import SoulseekAdapter
 from lyra_worker.adapters.youtube import YouTubeAdapter
 from lyra_worker.browser import MbBrowser
+from lyra_worker.probe import AudioProbe
 from lyra_worker.resolver import MbResolver
 from lyra_worker.tagger import Tagger
 
@@ -40,3 +42,8 @@ def build_tagger() -> Tagger:
 def build_browser() -> MbBrowser:
     """The MusicBrainz browser used by the background monitor."""
     return MusicBrainzBrowser()
+
+
+def build_probe() -> AudioProbe:
+    """The audio probe used by the library scan."""
+    return MutagenProbe()