feat: add CI build check and improve release workflow

[andrescera]

Summary

• Add build-check.yml: CI workflow that runs on PR/push to main
• Rename build-distributions.yml to publish-release.yml for consistency
• Update workflow to checkout sibling deps (ceracoder, srtla)

Features

• Builds backend for arm64/amd64, frontend
• Add SHA256 checksums for all release artifacts
• GPG signs APT repository metadata
• Uploads to Cloudflare R2

Test plan

• Verify build-check runs on PR
• Test publish-release workflow manually

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