Warn users when requested subtitle language != video's original language to avoid 429 on auto-translated subs

[juananpe]

Requesting auto-translated subtitles for a different language than the video's original language can trigger YouTube rate-limiting and cause our ingestion to fail with HTTP 429 (see yt-dlp/yt-dlp#13831). We should detect this case early and warn the user (or provide an explicit fallback) instead of failing with a 429 error.

Steps to reproduce

1. Attempt to ingest a YouTube video whose original language is Spanish.
2. Request subtitles in a different language (e.g., English auto-translated subs).
3. Observe ingestion fail with 429: Too Many Requests (see logs).

Current behavior

• Ingestion attempts to download auto-translated subtitles and fails with HTTP 429.
• The error surfaces as an exception and the ingest is marked failed without a helpful user-facing message.

Expected behavior

• Detect when the requested subtitle language differs from the video's original language or when the requested subs are auto-translated.
• Present a clear, actionable warning to the user (and optionally fail gracefully) before attempting to download the auto-translated subs.

Acceptance criteria

• When the mismatch is detected, the user sees a clear warning explaining the 429-risk and suggested actions.
• If possible add playwright e2e tests covering detection logic and the warning path (not mandatory)
  (see: https://github.com/Lamb-Project/lamb/tree/main/testing/playwright)

[LucasImpulse]

Does this problem cover autogenerated subtitles that are then auto-translated into other languages than the original? Potentially doesn't affect manual subtitles that the uploader themselves have specifically created for other languages. Used yt_dlp to fetch auto-translation examples, and it reveals that the original language subtitles are suffixed with "-orig" so perhaps on an English video, "en-orig" is fine but "es" or "fr" causes a 429, or a Spanish video "es-orig" is fine but "en" = 429 error. So at least the backend logic of identifying originals will be easy.

[juananpe]

In our tests auto-translated subtitles (the simple language codes like en, fr) often hit 429, while the original auto-generated captions have -orig (e.g. es-orig) and download fine; manual uploader subtitles do not appear affected.

[LucasImpulse]

In that case #222 for the backend should be ready, not sure if the user facing warning needs to go on the ingest plugin itself, query.py or the KB Svelte frontend.