GitLab and Bitbucket support

[fusion94]

Summary

Expand CLAHub beyond GitHub to support GitLab and Bitbucket repositories, merge requests, and authentication.

Motivation

Many organizations use GitLab (self-hosted or gitlab.com) or Bitbucket. Supporting multiple platforms significantly expands CLAHub's addressable market and makes it viable for multi-platform organizations.

Implementation notes

• Abstract the Git platform layer behind an interface (webhooks, status checks, user identity, API)
• GitLab: OAuth, merge request webhooks, commit status API
• Bitbucket: OAuth 2.0, PR webhooks, build status API
• Per-agreement platform selector
• Cross-platform identity linking (same contributor on GitHub + GitLab)
• This is a large effort — consider GitLab first as it's more common in enterprise

Phase

Phase 8: Polish & Expansion (v3.0 roadmap)