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>

web/src/lib/api/admin.ts

@@ -205,6 +205,12 @@ export type ScanStageCoverEnrich = {
   failed: number;
 };
 
+export type ScanStageArtistArtEnrich = {
+  processed: number;
+  succeeded: number;
+  failed: number;
+};
+
 export type ScanStatus = {
   id: string;
   started_at: string;
@@ -212,6 +218,7 @@ export type ScanStatus = {
   library?: ScanStageLibrary;
   mbid_backfill?: ScanStageMbidBackfill;
   cover_enrich?: ScanStageCoverEnrich;
+  artist_art_enrich?: ScanStageArtistArtEnrich;
   error_message?: string;
   in_flight: boolean;
 };