feat(server/m7-scan-progress): wire stagePublisher into 4 scan stages

Each of the 4 stage workers (library walk, MBID backfill, cover
enrich, artist art enrich) gains a progressCb parameter that
receives a snapshot of the current running state per item processed.
The orchestrator stores the snapshot in a local var that the
publisher's marshal closure reads at flush time.

scanrun.go's 4 stage blocks are rewritten to construct one publisher
per stage, pass a Tick-firing closure to the worker as progressCb,
and call Flush() after the worker returns (routing any persist error
to captureErr — preserves today's diagnostic behavior).

The previous post-stage UpdateScanRun* calls are removed; the
publisher's Flush handles the final write. Cadence: every 500 items
OR every 1s, whichever fires first.

Outside-orchestrator callers (scanner_test.go, enricher_test.go)
pass nil for progressCb — no-op behavior preserved for tests that
don't need progress observability.

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

internal/coverart/artist_enricher.go

@@ -135,7 +135,8 @@ 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.
-func (e *Enricher) EnrichArtistBatch(ctx context.Context, limit int, dataDir string) (processed, succeeded, failed int, err error) {
+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 {
 		return 0, 0, 0, nil
 	}
@@ -156,12 +157,18 @@ 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++
+			if progressCb != nil {
+				progressCb(processed, succeeded, failed)
+			}
 			continue
 		}
 		// Re-read to classify.
 		row, rerr := q.GetArtistByID(ctx, id)
 		if rerr != nil {
 			failed++
+			if progressCb != nil {
+				progressCb(processed, succeeded, failed)
+			}
 			continue
 		}
 		if row.ArtistArtSource != nil &&
@@ -171,6 +178,9 @@ func (e *Enricher) EnrichArtistBatch(ctx context.Context, limit int, dataDir str
 		} else {
 			failed++
 		}
+		if progressCb != nil {
+			progressCb(processed, succeeded, failed)
+		}
 	}
 	return processed, succeeded, failed, nil
 }

internal/coverart/enricher.go

@@ -146,7 +146,8 @@ 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.
-func (e *Enricher) EnrichBatch(ctx context.Context, limit int) (processed, succeeded, failed int, err error) {
+func (e *Enricher) EnrichBatch(ctx context.Context, limit int,
+	progressCb func(processed, succeeded, failed int)) (processed, succeeded, failed int, err error) {
 	if limit <= 0 {
 		return 0, 0, 0, nil
 	}
@@ -167,12 +168,18 @@ func (e *Enricher) EnrichBatch(ctx context.Context, limit int) (processed, succe
 			e.logger.Warn("coverart: batch entry failed",
 				"album_id", uuidString(id), "err", eerr)
 			failed++
+			if progressCb != nil {
+				progressCb(processed, succeeded, failed)
+			}
 			continue
 		}
 		// Re-read to classify.
 		row, rerr := q.GetAlbumWithFirstTrackPath(ctx, id)
 		if rerr != nil {
 			failed++
+			if progressCb != nil {
+				progressCb(processed, succeeded, failed)
+			}
 			continue
 		}
 		if row.CoverArtSource != nil &&
@@ -184,6 +191,9 @@ func (e *Enricher) EnrichBatch(ctx context.Context, limit int) (processed, succe
 		} else {
 			failed++
 		}
+		if progressCb != nil {
+			progressCb(processed, succeeded, failed)
+		}
 	}
 	return processed, succeeded, failed, nil
 }

internal/coverart/enricher_test.go

@@ -263,7 +263,7 @@ func TestEnrichBatch_DrainsNullSourceOnly(t *testing.T) {
 	}
 
 	e := newTestEnricher(t, pool)
-	processed, _, _, err := e.EnrichBatch(context.Background(), 100)
+	processed, _, _, err := e.EnrichBatch(context.Background(), 100, nil)
 	if err != nil {
 		t.Fatalf("batch: %v", err)
 	}