feat: cargo audit auto-fix job

[dandanlen]

Pull Request

A GH action that uses Claude Code to deal with cargo audit issues.

Triggered once daily (see cron), and on every merge to main, or manually.

TODO: sort out permissions:

1. Create a GitHub App (org or personal) with Contents: Write + Pull requests: Write permissions
2. Store the credentials as org-level (or repo-level) variable/secret:
   - vars.CI_APP_ID → the App's numeric ID
   - secrets.CI_APP_PRIVATE_KEY → the generated .pem key
3. Install the App on the chainflip-backend repo

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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