Academic: section-aware allotment, calendar time windows, PG/PhD gating, seat fix

[vikrantwiz02]

This pull request introduces several improvements and new features to the academic calendar and course registration system, primarily adding support for time-bound events in the academic calendar, improving the handling of batch and section assignments, and refactoring registration window checks for maintainability and flexibility. The changes also update course allocation logic and add new eligibility checks for PG/PhD enrollments.

Academic Calendar: Time-bound Events

• Added from_time and to_time fields to the Calendar model, updated related API endpoints (list_calendar, add_calendar, update_calendar), and included a migration to support events with specific start and end times. [1] [2] [3] [4] [5]

Registration Window Eligibility Refactor

• Refactored all course registration eligibility checks to support both date and time windows, centralizing logic in _check_registration_window and related helpers, and added support for PG/PhD registration windows.

Course Allocation and Section Assignment

• Updated batch change logic to automatically assign students to the appropriate section in the new batch, choosing the only or emptiest section, or raising an error if the batch has no sections set up.
• Updated course allocation and seat counting logic to include all registrations, removing previous exclusions for backlog slots. [1] [2] [3] [4]

PG/PhD Registration Enforcement

• Enforced registration window checks for PG/PhD thesis, seminar, teaching credit, and course request APIs, blocking POST requests outside the allowed window. [1] [2] [3] [4]

Other Improvements

• Improved course offering resolution logic in student course addition and allotment, and ensured instructor assignment is handled consistently. [1] [2] [3] [4]