- Add ScanWarning struct with FilePath/Phase/Err to ScanMetrics
- Add mutex-protected addWarning method for concurrent use
- Reclassify walk, extraction, commit, orphan, variant, FTS failures as warnings
- Update commitBatch to return only fatal tx.Commit errors
- Update cachedLinkArtist to check errors via database.IsUniqueViolation
- Update handleConfigUpdate to capture and log scan warning count