Track timeslots that are sent to subscribers

[MiniCodeMonkey]

I.e. a new table with timeslot_id and subscriber_id + timestamps.

Can be tracked here: https://github.com/MiniCodeMonkey/curbside/blob/master/app/Jobs/ScanChain.php#L157
(or maybe dispatched as an event and consumed by both this code and the notification)

This can be used to:

• Filter out timeslots that the subscriber has already seen, so they don't get the same timeslots sent out multiple times if they continue subscribing
• Measure how effective this service actually is :)