Opt-in local auth and expose video deps readiness

[groupthinking]

Ensure local developers can run the full FastAPI+Next pipeline without 503s and get /health readiness fields for yt-dlp deps.

[linear-code]

GRV-307

[github-actions]

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[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• Local full pipeline: auth opt-in + youtube deps (F6/F7) #1347

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