Pay queue for lear events

Implement the alt-queue over a series of steps:

- [x] step1 - add minimal _publish_ to sbc-pay
- [ ] step2 - add filtering to send events to the correct subject (eg. business payments to business-queues)
- [ ] step3 - add on/off to the sbc-pay publishing per event type (FFlags or some other mapping to deploy)
- [ ] step4 - after sbc-pay moves to a newer python, replace this with the sbc-queue library (also use this to add features to the library)

Start by reviewing the existing minimal _publish_ implementation in sbc-pay and the event types it handles. Trace how events are currently sent, then define the affected subjects and deployment controls needed for the remaining steps; done means filtering, per-event enablement, and the planned sbc-queue replacement are addressed.

Written by the indexing model from the issue text.