Add Timetable resource

[ngjunsiang]

Deliverables

• timetable resource in /api/v1

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Per discussion on 2026/02/04:

See https://github.com/nyjc-computing/nyxchange-timetable-v2/issues/18#issue-3894924632

• campus.api.timetable.set_current(<UUID>)
• campus.api.timetable.set_next(<UUID>)
• campus.api.timetable[timetable_uid].get(...)
• campus.api.timetable[timetable_uid].list(...)
• campus.api.timetable.new({venues: [...], lessongroups: [...], entries: [...]})
• campus.profile -> User
• campus.api.timetable.get_current() -> CampusID
• campus.api.timetable.get_next() -> CampusID
• campus.api.timetable[timetable_uid].list(user_id=...) -> list[TimetableEntry]