Refactor / Standardize Endpoints

As a software developer, I would like to see proper implementation of endpoints and adherence to RESTful guidelines.

Ex.
`GET` /attendance/hm => List of users eligible for HM attendance.
`POST` /attendance/hm => Endpoint for posting HM attendance.

Basically, make our endpoints make some semblance of sense.

Start by locating the Flask endpoint definitions and reviewing the existing attendance routes, especially the GET and POST examples for /attendance/hm. Document the current inconsistencies before changing them; done means the endpoints follow a consistent RESTful structure and existing behavior remains covered by the available tests.

Written by the indexing model from the issue text.