ci: build signed APKs for release-labeled PRs

[mister-teddy]

Summary

Adds release-signed APK artifacts for same-repository PRs targeting main when they carry exactly one app release label.

Details

• Adds a lightweight PR classifier job for app release labels.
• Runs the expensive signed APK build only for same-repo PRs with one of release:breaking, release:feature, release:bugfix, release:patch, release:improvement, or release:optimisation.
• Skips PR APK builds for release:docs.
• Uploads the signed release APK as an Actions artifact for qualifying PRs.
• Keeps tag release behavior intact, including APK, debug APK, AAB, manuals, and GitHub Release publishing.
• Documents the PR artifact flow in the developer build docs.

Validation

• git diff --check
• Parsed .github/workflows/build.yml with PyYAML

actionlint was not installed in the local environment, so I could not run it here.