feat(web): MB-resolve manual imports for instant cover art

A manually-imported album had no release-group MBID, so it showed the ◉
placeholder until a Scan matched it. Now the import resolves the release group
(searchReleaseGroup, via the shared MB cache) and stores it on the new
LibraryItem.rgMbid column (migration add_library_rg_mbid). The library route
prefers LibraryItem.rgMbid, falling back to the MonitoredRelease join for
scanned items — so a manual album gets cover art immediately. Best-effort: a
no-match / MB outage just leaves rgMbid null.

web 164 tests, tsc + build clean.

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

web/prisma/schema.prisma

@@ -82,6 +82,9 @@ model LibraryItem {
   format       String
   qualityClass Int
   importedAt   DateTime @default(now())
+  // MusicBrainz release-group MBID when known (resolved at manual import) — gives the album
+  // cover art without depending on a matching MonitoredRelease row.
+  rgMbid       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