Federation: routine to clear old activities

Overtime the number of stored activities piles up

They should be removed after a while in a routine.. @balessan @jbpasquier do you know if there is some Activity Pub guidance on the lifetime of an activity?

Update 07/05/2021

I looked into this one again and thought I should elaborate. The Linked Data Notifications spec refers to storing notifications and returning them

In the ActivityPub spec it states that

unless the activity is transient, MUST include the new id in the Location header ... The server MUST then add this new Activity to the outbox collection