fix(i18n): use messages catalog

[Samk13]

❤️ Thank you for your contribution!

Description

• Configure Babel in pyproject.toml for pyproject-based extraction and catalog compilation.

• Rename backend translation catalogs to the default messages domain expected by reusable CI workflows.

• Update Transifex config, runtime i18n domain, and docs to match the new catalog name.

• related: Track i18n work for InvenioRDM v14 invenio-app-rdm#3499

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've added relevant test cases.
• I've added relevant documentation.
• I've marked translation strings.
• I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• I've NOT included third-party code (copy/pasted source code or new dependencies).
  • If you have added third-party code (copy/pasted or new dependencies), please reach out to an architect on Discord.

Frontend

• I've followed the CSS/JS and React guidelines.
• I've followed the web accessibility guidelines.
• I've followed the user interface guidelines.

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[max-moser]

Thanks a ton for fixing what I missed; that looks good! 🚀