feat(server/m7-381): RunScan orchestrator chains library+backfill+enrich

Adds internal/library/scanrun.go with RunScan, which creates a scan_runs
row and sequences file-walk → MBID backfill → cover enrich, persisting
per-stage tallies as jsonb. Extends coverart.EnrichBatch to return
(processed, succeeded, failed, err) so the orchestrator can classify
outcomes. Replaces main.go's two separate boot goroutines with a single
RunScan call; passes nil scanner in the no-startup-scan branch.

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

internal/coverart/enricher.go

@@ -103,27 +103,45 @@ func (e *Enricher) EnrichAlbum(ctx context.Context, albumID pgtype.UUID) error {
 
 // EnrichBatch drains up to limit albums whose cover_art_source is NULL.
 // Iterates serially — the fetcher's rate limiter prevents bursts.
-func (e *Enricher) EnrichBatch(ctx context.Context, limit int) (int, error) {
+// Returns per-batch tallies for the orchestrator's scan-run record.
+//
+// "succeeded" means the album ended up with a non-NULL cover_art_source
+// other than 'none' (i.e. we found art via sidecar or MBCAA). "failed"
+// counts both EnrichAlbum errors and rows that ended at 'none'.
+func (e *Enricher) EnrichBatch(ctx context.Context, limit int) (processed, succeeded, failed int, err error) {
 	if limit <= 0 {
-		return 0, nil
+		return 0, 0, 0, nil
 	}
 	q := dbq.New(e.pool)
-	rows, err := q.ListAlbumsMissingCover(ctx, int32(limit))
-	if err != nil {
-		return 0, fmt.Errorf("list missing: %w", err)
+	rows, qerr := q.ListAlbumsMissingCover(ctx, int32(limit))
+	if qerr != nil {
+		return 0, 0, 0, fmt.Errorf("list missing: %w", qerr)
 	}
-	processed := 0
 	for _, r := range rows {
 		if ctx.Err() != nil {
-			return processed, ctx.Err()
-		}
-		if err := e.EnrichAlbum(ctx, r.ID); err != nil {
-			e.logger.Warn("coverart: batch entry failed",
-				"album_id", uuidString(r.ID), "err", err)
+			return processed, succeeded, failed, ctx.Err()
 		}
 		processed++
+		if eerr := e.EnrichAlbum(ctx, r.ID); eerr != nil {
+			e.logger.Warn("coverart: batch entry failed",
+				"album_id", uuidString(r.ID), "err", eerr)
+			failed++
+			continue
+		}
+		// Re-read the source to classify the outcome.
+		row, rerr := q.GetAlbumWithFirstTrackPath(ctx, r.ID)
+		if rerr != nil {
+			failed++
+			continue
+		}
+		if row.CoverArtSource != nil &&
+			(*row.CoverArtSource == "sidecar" || *row.CoverArtSource == "mbcaa" || *row.CoverArtSource == "embedded") {
+			succeeded++
+		} else {
+			failed++
+		}
 	}
-	return processed, nil
+	return processed, succeeded, failed, nil
 }
 
 // EnrichRetryMissing drains both NULL and 'none' rows. Used by the admin

internal/coverart/enricher_test.go

@@ -219,7 +219,7 @@ func TestEnrichBatch_DrainsNullSourceOnly(t *testing.T) {
 	}
 
 	e := NewEnricher(pool, discardLogger(), nil, false)
-	processed, err := e.EnrichBatch(context.Background(), 100)
+	processed, _, _, err := e.EnrichBatch(context.Background(), 100)
 	if err != nil {
 		t.Fatalf("batch: %v", err)
 	}