[BACKEND] Rewrite SSE endpoint to return a map of reservations per event id

[bcho892]

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

Right now in server\src\data-layer\services\EventService.ts we simply sum up all the total reservations. This may not be the most useful, instead we want the data returned in the format:

{
    "event_1": 200,
    "event_2": 20,
// Etc ...
}

So on the front end we can programatically match them with the relevant event.

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