B.Tech sectioning, per-section grade submission & course feedback - #1941

- Add StudentBatchUpload.section, auto-derived in save() from discipline +
  roll-number parity, so a student's section is decided when data is fed via
  Admin > Upcoming Batches (recomputed on every save, e.g. when a roll number
  is later assigned). Auto-exposed by StudentBatchUploadSerializer (fields '__all__').
- Migration: programme_curriculum 0034.

- get_batch_students now includes each student's section in its response so the
  Upcoming Batches admin view can display it (StudentBatchUpload.section, derived
  at admission from discipline + roll-number parity).

- New acadadmin Section Assignment API: section/batches, section/students, section/assign
- Stop auto-deriving Student.section (onboarding + StudentBatchUpload.save); branch change clears it
- get_batch_students shows the assigned section read-only (by roll number)
- available_courses exposes per-course sections from enrolled students' Student.section
- Faculty assigned-courses API returns each offering's section

- SubmitGradesProfAPI: allow acadadmin (lists all offerings that term) and
  return per-course sections (faculty = own offerings' section_labels, acadadmin
  = all); electives/interdisciplinary have none.
- UploadGradesProfAPI: allow acadadmin (bypass instructor ownership), accept an
  optional section to scope the roster; each grade still binds to the student's
  own offering via resolve_offering.
- download_template + PreviewGradesAPI: accept section to scope the roster.

student_questions now resolves each registered course's instructor via
resolve_offering (the student's own section offering), falling back to any
offering for single-offering / legacy no-section courses.

- FeedbackResponse.course_instructor FK (migration 0022) so responses are tied
  to the offering rated; student_submit binds it via the shown offering.
- student_questions: skip courses with no instructor; show the student's section instructor.
- admin_course_list: split sectioned courses into per-instructor rows (+ an
  "unassigned" row for responses with no offering recorded).
- admin_all_stats: filter by course_instructor (incl. isnull); fixed section order.

- GradeStatusAPI: report each offering by its own status; fall back to
  course-level only when no offering has grades bound (was marking unsubmitted
  sections as Submitted/Verified).
- download_template & PreviewGradesAPI: faculty may only access courses they
  teach (acadadmin/Dean: any); previously any faculty could pull any roster.

- UploadGradesProfAPI: reset reSubmit only for this submission's roster (never
  course-wide when programme_type is empty).
- student_submit: accept an option only if it belongs to the question, and
  derive the section from the question instead of trusting the request.

Bring sections up to date with the latest production while keeping its
section-aware / per-instructor grade-submission and B.Tech sectioning work.
Reconcile examination grade submission: keep section-aware roster scoping and
Summer CourseInstructor year handling, and adopt production's selected-role
acadadmin detection and registration-based acadadmin course list.

Sections are a UG-only concept (compute_section returns a section only for
B.Tech/B.Des). Gate all section handling on programme_type == UG in
download_template, UploadGradesProfAPI (section param + per-section roster
scoping) and PreviewGradesAPI, so PG/PhD grade submission ignores sections
and grades the whole programme's registered students together.

…lean