citegate v0.3.0

• DBLP added as a third verification source: search fallback for entries without a DOI that Crossref and OpenAlex miss (#2, PR #4 by @floze-the-genius)
• --only retractions fast mode: checks only the retraction status of DOI entries (two requests per entry), cheap enough to run daily; Action input only: 'retractions' (#1)
• Hardening from adversarial cross-review: an undetermined retraction status is now reported as error instead of verified; DBLP title markup is stripped; source-client robustness fixes
• README pre-commit rev bumped to v0.3.0