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

[lassebenni]

Problem

Every Grade Assignment run on this repo fails with startup_failure (0s, no jobs) — the autograder never runs.

Root cause: the grade job sets only issues: write + pull-requests: write. A permissions: block forces every unlisted scope to none, so this grants contents: none. The reusable HackYourFuture/github-actions/.github/workflows/auto-grade.yml@main requests contents: read (for actions/checkout), added 2026-05-03. GitHub rejects a reusable workflow needing more permission than the caller grants, at load time → startup_failure.

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

Note: this PR run still uses the base-branch (broken) workflow because pull_request_target evaluates from the base ref, so its own check still fails until merged. After merge, PRs grade correctly.

🤖 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.