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/api/admin_scan.go

@@ -17,14 +17,15 @@ import (
 // rather than base64 byte arrays — the DB column is bytea-shaped jsonb in
 // pgx but the data is already valid JSON.
 type scanStatusResp struct {
-	ID           string          `json:"id"`
-	StartedAt    string          `json:"started_at"`
-	FinishedAt   *string         `json:"finished_at"`
-	Library      json.RawMessage `json:"library,omitempty"`
-	MbidBackfill json.RawMessage `json:"mbid_backfill,omitempty"`
-	CoverEnrich  json.RawMessage `json:"cover_enrich,omitempty"`
-	ErrorMessage *string         `json:"error_message,omitempty"`
-	InFlight     bool            `json:"in_flight"`
+	ID              string          `json:"id"`
+	StartedAt       string          `json:"started_at"`
+	FinishedAt      *string         `json:"finished_at"`
+	Library         json.RawMessage `json:"library,omitempty"`
+	MbidBackfill    json.RawMessage `json:"mbid_backfill,omitempty"`
+	CoverEnrich     json.RawMessage `json:"cover_enrich,omitempty"`
+	ArtistArtEnrich json.RawMessage `json:"artist_art_enrich,omitempty"`
+	ErrorMessage    *string         `json:"error_message,omitempty"`
+	InFlight        bool            `json:"in_flight"`
 }
 
 // handleGetScanStatus implements GET /api/admin/scan/status.
@@ -62,6 +63,9 @@ func (h *handlers) handleGetScanStatus(w http.ResponseWriter, r *http.Request) {
 	if len(row.CoverEnrich) > 0 {
 		resp.CoverEnrich = row.CoverEnrich
 	}
+	if len(row.ArtistArtEnrich) > 0 {
+		resp.ArtistArtEnrich = row.ArtistArtEnrich
+	}
 	writeJSON(w, http.StatusOK, resp)
 }