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>

internal/db/queries/scan_runs.sql

@@ -27,11 +27,17 @@ UPDATE scan_runs
 -- name: GetLatestScanRun :one
 -- Powers GET /api/admin/scan/status. Returns NoRows when no scan has
 -- ever run; the API layer maps that to a 200 with empty payload.
-SELECT id, started_at, finished_at, library, mbid_backfill, cover_enrich, error_message
+SELECT id, started_at, finished_at, library, mbid_backfill, cover_enrich, artist_art_enrich, error_message
   FROM scan_runs
  ORDER BY started_at DESC
  LIMIT 1;
 
+-- name: UpdateScanRunArtistArtEnrich :exec
+-- M7 cover-sources: persists the artist-art enrich stage tally JSON.
+UPDATE scan_runs
+   SET artist_art_enrich = $2
+ WHERE id = $1;
+
 -- name: GetInFlightScanRun :one
 -- Used by POST /api/admin/scan/run to 409 when a scan is already running.
 -- "In flight" = finished_at IS NULL.