feat: Question variations support in assignment groups

[Copilot]

Assignment groups currently show every student the same set of assignments. This adds per-student variation support: a group can have a pool of assignments where each student is assigned only a subset (e.g., 2 of 4 possible questions).

Schema changes

• AssignmentGroup.variation_count (int, default 0) — when > 0, students complete this many variation groups instead of all assignments
• AssignmentGroupMembership.variation_group (nullable int) — NULL = required for everyone; positive integer = variation pool tag
• New assignment_group_variation table — records the specific (user, group, assignment) triples representing each student's assigned subset

Model logic

AssignmentGroup.get_assignments(user_id=None) is extended to filter by variation when user_id is supplied and variation_count > 0:

# Returns required assignments + this user's variation assignments
group.get_assignments(user_id=student.id)

# Returns all assignments (backward-compatible)
group.get_assignments()

New helpers on AssignmentGroup:

• get_variation_groups() — distinct variation pool numbers in use
• assign_variation_to_user(user_id, assignment_ids) — explicit per-student assignment
• assign_random_variation(user_id) — randomly selects variation_count pools and assigns

parse_assignment_load in controllers/helpers.py now passes user_id so students loading a group URL automatically see only their assigned subset.

New API endpoints (/assignment_group/)

Endpoint	Method	Purpose
/edit_variation_settings	GET/POST	Set variation_count and per-membership variation_group tags
/get_variations	GET	Read a user's assigned variation assignments
/assign_variation	POST	Instructor explicitly assigns specific assignments to a student
/assign_variations_random	POST	Bulk-randomize variation assignments for all enrolled students

Migration

Alembic migration b1c2d3e4f5a6 adds assignment_group.variation_count, assignment_group_membership.variation_group, and the assignment_group_variation table.