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/library/scanner.go

@@ -52,7 +52,10 @@ func New(pool *pgxpool.Pool, logger *slog.Logger, paths []string) *Scanner {
 // Scan walks every configured root and upserts any audio file whose mtime is
 // newer than the existing row's updated_at. Walk errors and per-file errors
 // are logged + counted; the scan keeps going.
-func (s *Scanner) Scan(ctx context.Context) (Stats, error) {
+//
+// progressCb (may be nil) receives the current Stats snapshot after each
+// processed file. Used by the orchestrator to drive partial-tally writes.
+func (s *Scanner) Scan(ctx context.Context, progressCb func(Stats)) (Stats, error) {
 	var stats Stats
 	q := dbq.New(s.pool)
 	start := time.Now()
@@ -65,6 +68,9 @@ func (s *Scanner) Scan(ctx context.Context) (Stats, error) {
 			if err != nil {
 				s.logger.Warn("library scan walk error", "path", path, "err", err)
 				stats.Errored++
+				if progressCb != nil {
+					progressCb(stats)
+				}
 				return nil
 			}
 			if d.IsDir() {
@@ -77,6 +83,9 @@ func (s *Scanner) Scan(ctx context.Context) (Stats, error) {
 				s.logger.Warn("library scan file error", "path", path, "err", err)
 				stats.Errored++
 			}
+			if progressCb != nil {
+				progressCb(stats)
+			}
 			return nil
 		}); err != nil {
 			return stats, fmt.Errorf("library: walk %q: %w", root, err)