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_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)
 	}