feat: classify AI errors — ValidationError→invalid, fatal/network→failed ...

Item 6 from REVIEW_FOLLOWUP:
- ai_normalizer: pydantic.ValidationError returns is_real_estate=False (invalid)
  OllamaFatalError / unexpected exceptions now raise → pipeline status=failed
- ai_enricher: ValidationError returns None (skip enrichment), fatal returns None,
  unexpected raise → pipeline status=failed
- pipeline._stage_enrich: propagate OllamaRetryableError so pipeline can mark failed

Co-Authored-By: Claude <noreply@anthropic.com>