Do not override Special Term II courses when migrating academic year

[ravern]

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

When NUSMods migrates from a previous academic year to a new one, Special Term II courses are also migrated, meaning that students taking the current Special Term II courses are left without access to course data on NUSMods.

For example, we migrated to a new academic year on 1 July 2025 but Special Term II only ends on 1 Aug 2025.

Describe the solution you'd like

Needs scope and discussion.

Describe alternatives you've considered

Unfortunately, the current solution by NUS is to refer students to a examination directory outside of NUSMods. This is disruptive for the students who now have multiple sources of information.

Additional context

Requested by NUS staff.

[jloh02]

A simple fix is to implement

1. Configuration for "previousSemester" instead of just current semester
2. Show exam schedules for previous special term on the respective courses page

this doesn't completely fix timetables. a more robust solution:

1. allow independent data switch for special term and normal semesters
2. add switching of special term AY data sometime after/during semester 1