feat(library): match own-state on artist MBID, not just name

Own-state ("in library") relied on an exact artist-NAME match, so an owned
album whose name differed from the MB canonical (punctuation/locale/feat.)
showed as not-owned even when followed. Capture the MusicBrainz artist MBID
on LibraryItem and prefer it for matching.

- New LibraryItem.artistMbid (migration add_library_artist_mbid). Populated at
  import (pipeline) + scan, both of which already hold the resolved target;
  the pipeline also now stores rgMbid (it previously didn't). COALESCE on
  conflict backfills MBIDs on re-scan without clobbering.
- /api/artists "in library, not followed" matches by artistMbid first, name
  case-insensitively as fallback (rows predating the column). Library route
  prefers the item's own artistMbid over the release join.

Last.fm/discover own-state still use name-match (they work) — a smaller
follow-up. worker 233, web 186 tests.

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

worker/tests/test_scan.py

@@ -49,9 +49,9 @@ def test_scan_records_have_monitored_and_follows_artist(conn, tmp_path):
     assert (result.imported, result.skipped) == (1, 0)
     assert _counts(conn) == {"LibraryItem": 1, "MonitoredRelease": 1, "WatchedArtist": 1}
     with conn.cursor() as cur:
-        cur.execute('SELECT source, "qualityClass" FROM "LibraryItem" WHERE artist=%s AND album=%s',
-                    ("John Mayer", "Continuum"))
-        assert cur.fetchone() == ("scan", 3)
+        cur.execute('SELECT source, "qualityClass", "artistMbid", "rgMbid" FROM "LibraryItem" '
+                    'WHERE artist=%s AND album=%s', ("John Mayer", "Continuum"))
+        assert cur.fetchone() == ("scan", 3, "a1", "rg1")  # MBIDs captured for own-state matching
         cur.execute('SELECT monitored, state, "currentQualityClass" FROM "MonitoredRelease" WHERE "rgMbid"=%s',
                     ("rg1",))
         assert cur.fetchone() == (True, "fulfilled", 3)