Backfill Ep Check Labels

Translate legacy compliance/ai_compliance values onto check_label/ ai_check_label so existing rows read correctly under the new label system. The legacy enum values are lowercase/snake_case (RequirementComplianceStatus), so each is mapped onto the matching DEFAULT_COMPLIANCE_LABELS text (or NO_EVIDENCE_LABEL) via _LEGACY_COMPLIANCE_TO_LABEL rather than written through verbatim. classified_at (the new completion sentinel) is backfilled from modified/created only for rows that were actually AI-classified (ai_compliance IS NOT NULL) — reviewer-only overrides with no AI run otherwise wouldn't have a sentinel and stay unclassified, matching live behavior.