Outbound activitypub send queue

There's an inbound queue implemented with bullmq and redis. An outbound queue would be a good addition. It shouldn't be too hard to do it naively, just throw the body of `sendActivity` into the worker and make the new `sendActivity` append to the queue instead.

Start with the existing BullMQ/Redis inbound queue and the sendActivity entry point; trace how work reaches the worker. The work is done when outbound sendActivity calls append to a queue and the worker performs the existing sendActivity body.

Written by the indexing model from the issue text.