#15586 add manual reversal credit flow

Adds an undo-flow on the backend to reverse manually granted credits

What can be reversed

• Only manual credits, benefit credits are managed by the benefit system.
• Within 7 days. A bit arbitrary, but reason is for one-time products that have no billing cycle to limit them.
• Current billing period only and not if already invoiced
• Once per credit, so that retries can't take more than was granted etc

How much is reversed

• Only the unused part. A reversal must never put the customer into an unexpected overage.
• The balance is recalculated: the stored balance can be stale.

The reversal event

• A new negative meter.credited event
• The reason stays "manual", with reversed_event_id pointing to the credit
• The rollover flag is copied, units come out of the same place they went into.
• Own event instead of parent event, since its a system event and only user events have parent events