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/probe.py

@@ -0,0 +1,16 @@
+from dataclasses import dataclass
+from typing import Protocol
+
+
+@dataclass(frozen=True)
+class ProbeResult:
+    artist: str
+    album: str
+    fmt: str
+    quality_class: int
+
+
+class AudioProbe(Protocol):
+    def probe(self, path: str) -> ProbeResult:
+        """Read an audio file's artist/album tags + format/quality."""
+        ...