GitLab integration: OAuth for GitLab.com

[emyller]

Context: #7000

The GitLab integration (#7159) supports token-based authentication. OAuth would add a one-click authorisation flow as an alternative for GitLab.com users.

Flagsmith can register an OAuth application on gitlab.com that any user can authorise against. Scope: api.

Scope

• Register a Flagsmith OAuth application on gitlab.com.
• Implement OAuth signature + callback endpoints. Store and refresh tokens.
• Access tokens expire in 2 hours. Refresh token lifetime is undocumented — handle refresh failure gracefully and allow re-authorisation without losing existing configuration.

Acceptance criteria

• GitLab.com users can authorise Flagsmith with one click as an alternative to pasting a token
• OAuth tokens are stored and refreshed transparently
• Failed refresh allows re-authorisation without losing linked issues/MRs