feat(server/m7-381): RunScan orchestrator chains library+backfill+enrich

Adds internal/library/scanrun.go with RunScan, which creates a scan_runs
row and sequences file-walk → MBID backfill → cover enrich, persisting
per-stage tallies as jsonb. Extends coverart.EnrichBatch to return
(processed, succeeded, failed, err) so the orchestrator can classify
outcomes. Replaces main.go's two separate boot goroutines with a single
RunScan call; passes nil scanner in the no-startup-scan branch.

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

cmd/minstrel/main.go

@@ -83,32 +83,34 @@ 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)
-		}
-	}()
-
+	// One unified scan chain: library walk → MBID backfill → cover enrich.
+	// Boot-time scan and manual-trigger scans share this path; results land
+	// in scan_runs for the admin overview.
 	if cfg.Library.ScanOnStartup && len(cfg.Library.ScanPaths) > 0 {
 		go func() {
-			if _, err := scanner.Scan(ctx); err != nil {
-				logger.Error("startup scan failed", "err", err)
-			}
-			// Enrich any newly-imported albums (and resume any partial backlog).
-			if _, err := coverEnricher.EnrichBatch(ctx, cfg.Library.CoverArtBackfillCap); err != nil {
-				logger.Warn("post-scan cover enrichment failed", "err", err)
+			if _, err := library.RunScan(ctx, pool, scanner, coverEnricher,
+				logger.With("component", "scan_run"),
+				library.RunScanConfig{
+					BackfillCap: 5000,
+					EnrichCap:   cfg.Library.CoverArtBackfillCap,
+				},
+			); err != nil {
+				logger.Warn("startup scan run failed", "err", err)
 			}
 		}()
 	} else {
-		// No startup scan — still run the boot backfill once for any rows
-		// the previous run didn't get to.
+		// No startup file walk, but still run backfill + enrich so cover
+		// progress doesn't stall just because the operator disabled
+		// scan-on-startup.
 		go func() {
-			if _, err := coverEnricher.EnrichBatch(ctx, cfg.Library.CoverArtBackfillCap); err != nil {
-				logger.Warn("boot cover enrichment failed", "err", err)
+			if _, err := library.RunScan(ctx, pool, nil, coverEnricher,
+				logger.With("component", "scan_run"),
+				library.RunScanConfig{
+					BackfillCap: 5000,
+					EnrichCap:   cfg.Library.CoverArtBackfillCap,
+				},
+			); err != nil {
+				logger.Warn("boot-only scan run failed", "err", err)
 			}
 		}()
 	}