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/adapters/fakes.py

@@ -1,6 +1,7 @@
 from typing import Callable
 
 from lyra_worker.browser import ArtistHit, ReleaseGroupInfo
+from lyra_worker.probe import ProbeResult
 from lyra_worker.types import Candidate, DownloadResult, MBTarget, Quality
 
 _QUALITIES = {
@@ -81,3 +82,11 @@ class FakeMbBrowser:
 
     def browse_release_groups(self, artist_mbid: str) -> list[ReleaseGroupInfo]:
         return list(self._releases.get(artist_mbid, []))
+
+
+class FakeAudioProbe:
+    """In-memory AudioProbe for tests."""
+    def __init__(self, artist="", album="", fmt="FLAC", quality_class=3):
+        self._r = ProbeResult(artist=artist, album=album, fmt=fmt, quality_class=quality_class)
+    def probe(self, path: str) -> ProbeResult:
+        return self._r