DeepL: Send authKey in HTTP header when fetching languages

[jermy-c]

• Send authKey in HTTP header when fetching languages
• https://developers.deepl.com/docs/resources/breaking-changes-change-notices/november-2025-deprecation-of-legacy-auth-methods
• Was causing an error in our project Mustang: Translation: Auto-Translate DeepL fetch languages fails mustang-im/mustang#1007

[benbucksch]

Tested and works for me.

Without this fix: FAIL https://github.com/mustang-im/mustang/actions/runs/21455272274/job/61794374317

Could not fetch supported languages from DeepL
Error: Could not fetch supported languages from DeepL
    at DeepL.fetchLanguages (/home/runner/work/mustang/mustang/app/node_modules/mustang-autotranslate/lib/services/deepl.js:97:19)
    at process.processTicksAndRejections (node:internal/process/task_queues:103:5)
    at async DeepL.initialize

With this fix: PASS https://github.com/mustang-im/mustang/actions/runs/21457865672

[leolabs]

Thank you for your PR! I'll push this in a new version soon :)