fix(ci): grant contents:read so the grade workflow stops failing at startup

[lassebenni]

Problem

Every Grade Assignment run fails with startup_failure (0s, no jobs). The grade job grants only issues: write + pull-requests: write, which forces contents: none. The reusable HackYourFuture/github-actions/.github/workflows/auto-grade.yml@main needs contents: read (checkout), added 2026-05-03, so GitHub rejects it at load time.

Fix: add contents: read to the grade job permissions (matches the working HackYourAssignment/c55-data-week-6).

Note: pull_request_target evaluates the workflow from the base ref, so this PR run still uses the broken version and fails until merged.

🤖 Generated with Claude Code

[lassebenni]

Closing: the backend track is out of scope for this pass (data-track work only). The change here is just the valid contents: read CI one-liner, identical to the data-track fix, but it should be applied and merged by the backend-track owners rather than from this data-track sweep. No changes retained.