Backend: Clear unpaid singups
There is only a limited time window to paid for a signup. After the window, the signup should be removed.
The big question is: When to check for this? Using some periodic task (requires extra setup)? Or triggered by an incoming request (might delay this request)?
I guess whenever a signup runs out of time, emails should be sent:
- to whoever lost the spot
- to whoever got the sport, if anybody was on the waiting list