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>

web/prisma/schema.prisma

@@ -89,6 +89,10 @@ model LibraryItem {
   // MusicBrainz release-group MBID when known (resolved at manual import) — gives the album
   // cover art without depending on a matching MonitoredRelease row.
   rgMbid       String?
+  // MusicBrainz ARTIST MBID when known (captured at import + scan). Own-state ("in library")
+  // prefers this over the fragile exact artist-name match; null rows fall back to name-match
+  // (re-scan to populate).
+  artistMbid   String?
   // The album's on-disk audio filenames ("## Title.ext"), captured at import + scan, so the
   // Library modal can show the REAL tracks on disk (revealing incomplete/edition mismatches)
   // instead of MusicBrainz's canonical listing. Empty for items imported before this existed