Cancelling a ticket should make the ticket not disappear completely if it was used already.

[Narigo]

Description
The current state of cancelling a ticket removes it completely from the user. If the slot already began, the ticket should still be available though.

Links/References

• Ticket Let users cancel a ticket #22 for the initial implementation of the cancellation
• Ticket Manage versions of the database #48 to make database changes possible

Tasks

• Make database versions possible Manage versions of the database #48
• Add a nullable deleted_at column in table ticket
• Implement the backend change to mark tickets as deleted
• Implement the backend change to not hand out tickets that are marked as deleted

Additional Context
Frontend should work already thanks to #22