cubicweb-mediaplayer #10413864 Use celery to make encoding async [validation pending]

prioritynormal
typeenhancement
done in0.2.0
load2.000
load left0.000
closed by#c182ddcfd5dd [celery] Replace synchronous encoding hooks by celery machinery
patch[celery] Replace synchronous encoding hooks by celery machinery [applied][celery] Replace synchronous encoding hooks by celery machinery [folded][doc,celery] Add worker/celery installation and configuration notes [folded][celery] Replace synchronous encoding hooks by worker/celery machinery [folded][schema,celery] Add relation from worker task to the media entity to be encoded [folded]