fix(thumbnails): rerender unverifiable registry artifacts

[raylim]

Summary

• Reuse an existing thumbnail only when its source fingerprint exactly matches current inventory.
• Force rerender for null or mismatched fingerprints.
• Require complete v2 tile metadata when selecting rows for publication.

Validation

• .venv/bin/python -m pytest -q — 198 passed

This is a prerequisite for the strict fingerprint-bound manifest update in pathology-data-mining/pdm_databricks_pipelines#24.

[raylim]

Consolidated into #38 so there is one active tile-server WSI PR.