feat(server,web/m7-cover-sources): scan-orchestrator 4th stage

Adds an artist-art-enrich stage after cover-enrich in RunScan. The
stage drains EnrichArtistBatch with a configurable cap and the
operator's data_dir, persists processed/succeeded/failed tallies to
the scan_runs.artist_art_enrich jsonb column added in migration
0018.

The admin /admin Library Scan card grid expands from 3 to 4 columns
with the new "Artist art" card mirroring the cover-enrichment one
(Processed / Succeeded / Failed). The TS ScanStatus type gains an
optional artist_art_enrich field. The Go scanStatusResp gains the
matching ArtistArtEnrich json.RawMessage field.

cmd/minstrel/main.go (and admin scan-trigger handler) wire the new
RunScanConfig fields: ArtistEnrichCap reuses cfg.Library.CoverArtBackfillCap
for now (can be split if separate budgets are useful), DataDir is
cfg.Storage.DataDir.

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

cmd/minstrel/main.go

@@ -100,8 +100,10 @@ func run() error {
 			if _, err := library.RunScan(ctx, pool, scanner, coverEnricher,
 				logger.With("component", "scan_run"),
 				library.RunScanConfig{
-					BackfillCap: 5000,
-					EnrichCap:   cfg.Library.CoverArtBackfillCap,
+					BackfillCap:     5000,
+					EnrichCap:       cfg.Library.CoverArtBackfillCap,
+					ArtistEnrichCap: cfg.Library.CoverArtBackfillCap,
+					DataDir:         cfg.Storage.DataDir,
 				},
 			); err != nil {
 				logger.Warn("startup scan run failed", "err", err)
@@ -115,8 +117,10 @@ func run() error {
 			if _, err := library.RunScan(ctx, pool, nil, coverEnricher,
 				logger.With("component", "scan_run"),
 				library.RunScanConfig{
-					BackfillCap: 5000,
-					EnrichCap:   cfg.Library.CoverArtBackfillCap,
+					BackfillCap:     5000,
+					EnrichCap:       cfg.Library.CoverArtBackfillCap,
+					ArtistEnrichCap: cfg.Library.CoverArtBackfillCap,
+					DataDir:         cfg.Storage.DataDir,
 				},
 			); err != nil {
 				logger.Warn("boot-only scan run failed", "err", err)
@@ -154,8 +158,10 @@ func run() error {
 	}
 
 	scanCfg := library.RunScanConfig{
-		BackfillCap: 5000,
-		EnrichCap:   cfg.Library.CoverArtBackfillCap,
+		BackfillCap:     5000,
+		EnrichCap:       cfg.Library.CoverArtBackfillCap,
+		ArtistEnrichCap: cfg.Library.CoverArtBackfillCap,
+		DataDir:         cfg.Storage.DataDir,
 	}
 	srv := server.New(logger, pool, scanner, subsonic.Config{
 		AllowPlaintextPassword: cfg.Subsonic.AllowPlaintextPassword,