feat(server/m7-380): one-shot MBID backfill worker for existing libraries

Adds a boot-time goroutine that walks albums with NULL mbid, re-reads
tags from one track per album via dhowden/tag, and persists album + artist
MBIDs. Healed albums also get their cover_art_source='none' cleared so the
enricher's next batch retries the MBCAA fetch. Caps at 5000 albums per
boot to avoid stalling startup on huge libraries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/minstrel/main.go

@@ -83,6 +83,16 @@ func run() error {
 	})
 	coverEnricher := coverart.NewEnricher(pool, logger.With("component", "coverart"), coverFetcher, cfg.Library.CoverArtFromMBCAA)
 
+	// One-shot MBID backfill: walks albums with NULL mbid, reads tags from
+	// one track per album, persists the MBID. Filling in MBIDs unblocks the
+	// cover enricher for libraries that were imported before scanner-side
+	// MBID extraction landed (M7 #380).
+	go func() {
+		if _, err := library.BackfillMBIDs(ctx, pool, logger.With("component", "mbid_backfill"), 5000); err != nil {
+			logger.Warn("mbid backfill failed", "err", err)
+		}
+	}()
+
 	if cfg.Library.ScanOnStartup && len(cfg.Library.ScanPaths) > 0 {
 		go func() {
 			if _, err := scanner.Scan(ctx); err != nil {