CD workflows: pin GitHub Actions to @v4 instead of @master

[thehabes]

Summary

Our CD workflows pin GitHub Actions to the moving @master branch instead of a stable major-version tag like @v4. Every workflow run silently picks up whatever HEAD of actions/checkout, actions/setup-node, and actions/cache happens to be — there is no diff in this repo when those upstream branches change, so a green PR yesterday can fail today for reasons we cannot see.

Affected workflows

• .github/workflows/cd_dev.yaml — 5 occurrences (3× actions/checkout@master, 1× actions/setup-node@master, 1× actions/cache@master)
• .github/workflows/cd_prod.yaml — 2 occurrences of actions/checkout@master

Our newer workflows (tests.yaml, shared_openapi_sync.yaml) already pin actions/checkout@v4 and actions/setup-node@v4, so the inconsistency is internal to this repo too.

Why this matters

• Reproducibility — same YAML produces different behavior over time.
• Supply-chain surface — a malicious or accidental commit to actions/checkout's master branch propagates to every run immediately.
• Breaking changes — when actions/checkout@v5 ships, @master consumers get the breakage at the next run with no warning.
• GitHub's own guidance explicitly recommends pinning to a major version tag or a SHA. Major-version tags (@v4) still receive patch/security updates automatically.

Cross-repo alignment

• RERUM v1 (CenterForDigitalHumanities/rerum_server_nodejs) already uses @v4 across all CI/CD workflows.
• TPEN-services has the same drift — tracked in CI/CD: pin GitHub Actions to @v4 instead of @master TPEN-services#525.
• TinyPEN has the same drift — tracked in CD workflows: pin actions/checkout to @v4 instead of @master TinyPen#50.

Proposed fix

Normalize the two CD workflows to:

• actions/checkout@v4
• actions/setup-node@v4
• actions/cache@v4

Small, contained sweep; no behavior changes expected (these are the same versions the existing tests.yaml and shared_openapi_sync.yaml already run on).

Acceptance criteria

• No @master references remain in .github/workflows/.
• All actions/checkout, actions/setup-node, actions/cache references in CD workflows pin to @v4.
• CI passes on the sweep PR.

Out of scope

• Pinning to commit SHAs — first-party actions/* major-version tags are sufficient.
• TPEN-services and TinyPEN cleanups — separate issues.