[BACKEND] [WEBHOOK] Expire all other checkout sessions involving date if it takes up last spot

[bcho892]

Is your feature request related to a problem? Please describe.

We check if there is someone in a session for a payment on stripe, however we do not account for the possible race condition when 2+ customers could theoretically get into a checkout session for the last spot on the date.

To reduce the effect of this we implement first come first serve - that is once the last spot has been taken by someone we want to expire all checkout sessions involving that date.

Note that this is probably not going to happen, and even with this change race conditions are possible - this is just a safeguard

BEFORE MERGING

• [ ] Integration test written for services
• [ ] Schemas annotated if adding new models
• [ ] Code generation run (hint: yarn workspace server tsoa spec-and-routes)
• [ ] Appropriate mocks created where possible
• [ ] PR Reviewed (For non-trivial changes)
• [ ] Changes tested after rebasing on master or merging in master (hint: git fetch origin master:master, then git rebase master or git merge master)
• [ ] All required PR checks passing