feat(server/coverart): batch summary log with category breakdown

The existing processed/succeeded/failed tally written to scan_runs
collapses too much to debug "0 successes" symptoms — failed includes
"settled to none (correct)" alongside "left NULL (transient/IO error,
will retry)" alongside "skipped (no MBID)". An operator looking at
{"failed": 144, "succeeded": 0} can't tell whether the enricher is
broken, the upstream is broken, the data is missing MBIDs, or the
artists genuinely aren't in TheAudioDB.

Adds a single end-of-batch Info log per stage with the breakdown:
  - eligible:    rows returned by the SQL filter
  - processed:   rows the loop actually touched
  - succeeded:   art attached (provider success)
  - settled_none: tried, no provider had art (legit miss)
  - no_mbid:     skipped inside the enricher because MBID was NULL
                 (artist stage only)
  - left_null:   tried, transient/IO failure, will retry next pass
  - errored:     internal error during the entry (logged separately)

The JSON written to scan_runs keeps its existing shape so the admin
UI's stage badges aren't disturbed.

internal/coverart/artist_enricher.go

@@ -135,6 +135,12 @@ func (e *Enricher) recordArtistArt(ctx context.Context, artistID pgtype.UUID, th
 // EnrichArtistBatch drains up to limit artists whose eligibility loop
 // determines they need processing. Iterates serially. Returns
 // per-batch tallies for the orchestrator's scan-run record.
+//
+// Emits a single Info log line at end-of-batch with a category
+// breakdown (succeeded / settled-none / no-MBID / left-NULL /
+// errored). The JSON tally written to scan_runs keeps the existing
+// processed/succeeded/failed shape for backwards compat with the
+// admin overview UI; the log line is the operator's diagnostic.
 func (e *Enricher) EnrichArtistBatch(ctx context.Context, limit int, dataDir string,
 	progressCb func(processed, succeeded, failed int)) (processed, succeeded, failed int, err error) {
 	if limit <= 0 {
@@ -148,8 +154,10 @@ func (e *Enricher) EnrichArtistBatch(ctx context.Context, limit int, dataDir str
 	if qerr != nil {
 		return 0, 0, 0, fmt.Errorf("list missing artist art: %w", qerr)
 	}
+	var settledNone, noMBID, leftNull, errored int
 	for _, id := range rows {
 		if ctx.Err() != nil {
+			e.logArtistBatchSummary(len(rows), processed, succeeded, settledNone, noMBID, leftNull, errored)
 			return processed, succeeded, failed, ctx.Err()
 		}
 		processed++
@@ -157,6 +165,7 @@ func (e *Enricher) EnrichArtistBatch(ctx context.Context, limit int, dataDir str
 			e.logger.Warn("coverart: artist batch entry failed",
 				"artist_id", uuidString(id), "err", eerr)
 			failed++
+			errored++
 			if progressCb != nil {
 				progressCb(processed, succeeded, failed)
 			}
@@ -166,25 +175,49 @@ func (e *Enricher) EnrichArtistBatch(ctx context.Context, limit int, dataDir str
 		row, rerr := q.GetArtistByID(ctx, id)
 		if rerr != nil {
 			failed++
+			errored++
 			if progressCb != nil {
 				progressCb(processed, succeeded, failed)
 			}
 			continue
 		}
-		if row.ArtistArtSource != nil &&
+		switch {
+		case row.ArtistArtSource != nil &&
 			*row.ArtistArtSource != "" &&
-			*row.ArtistArtSource != "none" {
+			*row.ArtistArtSource != "none":
 			succeeded++
-		} else {
+		case row.ArtistArtSource != nil && *row.ArtistArtSource == "none":
+			settledNone++
+			failed++
+		case row.Mbid == nil || *row.Mbid == "":
+			noMBID++
+			failed++
+		default:
+			leftNull++
 			failed++
 		}
 		if progressCb != nil {
 			progressCb(processed, succeeded, failed)
 		}
 	}
+	e.logArtistBatchSummary(len(rows), processed, succeeded, settledNone, noMBID, leftNull, errored)
 	return processed, succeeded, failed, nil
 }
 
+// logArtistBatchSummary emits a single Info line at end-of-batch with
+// the full category breakdown including MBID-skipped (artists eligible
+// by the SQL filter but skipped inside EnrichArtist due to NULL MBID).
+func (e *Enricher) logArtistBatchSummary(eligible, processed, succeeded, settledNone, noMBID, leftNull, errored int) {
+	e.logger.Info("coverart: artist enrichment batch complete",
+		"eligible", eligible,
+		"processed", processed,
+		"succeeded", succeeded,
+		"settled_none", settledNone,
+		"no_mbid", noMBID,
+		"left_null", leftNull,
+		"errored", errored)
+}
+
 // CleanupArtistArt removes the artist-art directory for a deleted
 // artist. Idempotent — no-op if the directory doesn't exist. Called
 // from the artist-delete cascade in tracks/service.go after the

internal/coverart/enricher.go

@@ -168,6 +168,12 @@ func (e *Enricher) recordAlbumCover(ctx context.Context, albumID pgtype.UUID, pa
 // EnrichBatch drains up to limit albums whose eligibility loop
 // determines they need processing. Iterates serially. Returns
 // per-batch tallies for the orchestrator's scan-run record.
+//
+// Emits a single Info log line at end-of-batch with a category
+// breakdown (succeeded / settled-none / left-NULL / errored). The
+// JSON tally written to scan_runs keeps the existing
+// processed/succeeded/failed shape for backwards compat with the
+// admin overview UI; the log line is the operator's diagnostic.
 func (e *Enricher) EnrichBatch(ctx context.Context, limit int,
 	progressCb func(processed, succeeded, failed int)) (processed, succeeded, failed int, err error) {
 	if limit <= 0 {
@@ -181,8 +187,10 @@ func (e *Enricher) EnrichBatch(ctx context.Context, limit int,
 	if qerr != nil {
 		return 0, 0, 0, fmt.Errorf("list missing: %w", qerr)
 	}
+	var settledNone, leftNull, errored int
 	for _, id := range rows {
 		if ctx.Err() != nil {
+			e.logBatchSummary("album", len(rows), processed, succeeded, settledNone, leftNull, errored)
 			return processed, succeeded, failed, ctx.Err()
 		}
 		processed++
@@ -190,6 +198,7 @@ func (e *Enricher) EnrichBatch(ctx context.Context, limit int,
 			e.logger.Warn("coverart: batch entry failed",
 				"album_id", uuidString(id), "err", eerr)
 			failed++
+			errored++
 			if progressCb != nil {
 				progressCb(processed, succeeded, failed)
 			}
@@ -199,27 +208,49 @@ func (e *Enricher) EnrichBatch(ctx context.Context, limit int,
 		row, rerr := q.GetAlbumWithFirstTrackPath(ctx, id)
 		if rerr != nil {
 			failed++
+			errored++
 			if progressCb != nil {
 				progressCb(processed, succeeded, failed)
 			}
 			continue
 		}
-		if row.CoverArtSource != nil &&
+		switch {
+		case row.CoverArtSource != nil &&
 			(*row.CoverArtSource == "sidecar" ||
 				*row.CoverArtSource == "mbcaa" ||
 				*row.CoverArtSource == "embedded" ||
-				*row.CoverArtSource == "theaudiodb") {
+				*row.CoverArtSource == "theaudiodb"):
 			succeeded++
-		} else {
+		case row.CoverArtSource != nil && *row.CoverArtSource == "none":
+			settledNone++
+			failed++
+		default:
+			leftNull++
 			failed++
 		}
 		if progressCb != nil {
 			progressCb(processed, succeeded, failed)
 		}
 	}
+	e.logBatchSummary("album", len(rows), processed, succeeded, settledNone, leftNull, errored)
 	return processed, succeeded, failed, nil
 }
 
+// logBatchSummary emits a single Info line at end-of-batch with the
+// full category breakdown. Operator-side diagnostic; the
+// processed/succeeded/failed tally collapses too much to debug
+// "0 successes" symptoms by itself.
+func (e *Enricher) logBatchSummary(stage string, eligible, processed, succeeded, settledNone, leftNull, errored int) {
+	e.logger.Info("coverart: enrichment batch complete",
+		"stage", stage,
+		"eligible", eligible,
+		"processed", processed,
+		"succeeded", succeeded,
+		"settled_none", settledNone,
+		"left_null", leftNull,
+		"errored", errored)
+}
+
 // EnrichRetryMissing drains both NULL and 'none' rows. Used by the
 // admin bulk-retry endpoint. Clears 'none' to NULL first so
 // EnrichAlbum picks them up.