Add Assignment Summary Report

New per-assignment analytics report for instructors showing per-question
stats (interactions, first/last touch, correct/incorrect/never tried, avg
score) with one column per student and a drilldown to individual student
detail. Scores sourced from question_grades table. Instructors excluded
from student columns. Background task + Redis polling pattern matches the
existing subchapoverview report.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: bnmnetp