fix(ci): What's New changelog data is not generated for release/development images #1865

What: Release and development images embed the empty placeholder backend/internal/changelog/data/changelog.json ([]), so the What's New modal never appears on stable releases. Only nightly-build.yml:263 runs scripts/generate-changelog.sh.

Where: .github/workflows/docker-build.yml (no generate-changelog step). The step was added in 8cefab1 and appears to have been lost in the 2026-08-10 merges (a02f621 / 1ef633c).

Severity: medium

Suggested approach: Generate the data in the setup job and pass it to the arm64/amd64 build jobs (they check out without history). Handle the release build possibly starting before release-please has created the vX.Y.Z tag.

Found while investigating the dev container's missing What's New modal; see docs/plans/current_spec.md (F1).