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>
bvandeusen