[BACKEND] [DEFECT] Adjust logic behind `validateUserRemaningSessions` for sourcing capacities

[jji05]

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

There is a major issue with validateUserRemaningSessions where if a user conveniently books a session that downgrades their membership status to casual, the capacity counting function validateUserRemaningSessions would give a false casual/member capacity due to the system counting capacities based on user roles.

Describe the solution you'd like

We need to update the booking schema to determine if a booking takes a casual or member slot.

Describe alternatives you've considered

This is open for discussion as there are several ways this user story could be completed.

Additional context

Reproduction steps:

1. Have an account with 1 booking session and member status
2. Book a session(which downgrades you to casual)
3. Check admin panel for capacities (you will see that there is a casual capacity increase and the normal capacity is left untouched)

BEFORE MERGING

• Integration test written for services
• Schemas annotated if adding new models
• Code generation run (hint: pnpm generate:types)
• 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