Fix QC playlist cleanup

[salmanfarisvp]

Summary

• fetch label associations before deleting a QC playlist
• unlink each association using both label_id and playlist_id, as required by the v4.1 API
• delete the playlist without explicitly deleting its playlist items

Root cause

The API rejects label-association deletion when only playlist_id is supplied. The cleanup returned early after that 400 response, so every scheduled run created another QC playlist without removing the previous one.

Scope

Cleanup continues to select playlists whose titles start with the QC prefix. Other playlist titles are not selected.

Test plan

• Validated Python syntax with ast.parse
• Checked the diff for whitespace errors
• Confirmed the corrected API sequence against the account by removing 165 accumulated QC playlists with zero failures
• Run the automated QC workflow and confirm the previous QC playlist is removed before the next one is created