[EmilienLDN]

Add a captions badge on video thumbnails

Fixes #7826

As discussed in the issue, the badge is only displayed when the video has at least one caption in the user's interface language (a Spanish caption is not useful for someone who only reads French).

What changed

Server

• The videos lists now return captionLanguages (e.g. ["ar", "zh"])
• No database migration: languages are read from the existing videoCaption table

Client

• A "CC" badge is displayed on the video thumbnail when one of the caption languages matches the interface language (fr-FR matches fr, zh-Hans-CN matches zh)
• The badge is also announced to screen readers and works in RTL

Tests

• Unit tests for the new hasLanguageForLocale helper
• API tests checking captionLanguages in the videos lists (no caption, two captions, after a caption deletion)

Notes

• captionLanguages is not returned by the video details endpoint, nor in playlist elements
• openapi.yaml is not updated yet
• I could not run the API tests locally (no PostgreSQL/Redis), so they still need to run in the CI

[Chocobozzz]

Thank you