Fix 500 on student_quiz_report_v2 when a subject is fully skipped

[pritamps]

Summary

Production hit a 500 on https://reports.avantifellows.org/reports/student_quiz_report/PunjabStudents_69d5060a9be5a82f7e85209e/204716. CloudWatch trace:

File "templates/student_quiz_report_v2.html", line 186
    <span class="font-semibold text-{{ color }}-600">{{ "%.1f"|format(subject.accuracy) }}%</span>
TypeError: must be real number, not NoneType

The DynamoDB item for that student has subject_performance[1].accuracy = NULL for Maths — the student skipped all 25 questions, so accuracy = correct / (correct + wrong) = 0/0 is mathematically undefined and the upstream pipeline persists it as DynamoDB NULL rather than 0. Jinja's "%.Nf"|format(...) then explodes.

Fix (two layers)

• render_v2_report() in app/routers/student_quiz_reports.py: coerce None → 0 for percentage and accuracy on both overall_performance and each entry of subject_performance before passing the payload to the template. This is the primary fix — any consumer of the v2 payload now sees clean numerics.
• student_quiz_report_v2.html and student_quiz_report_v2_print.html: every "%.Nf"|format(report.overall_performance.{percentage,accuracy}) and "%.Nf"|format(subject.{percentage,accuracy}) call now reads ... or 0 as a defense-in-depth fallback. The template can't crash on these fields even if a future code path skips the renderer sanitization.

Scope note: only the v2 + v2_print templates have "%.Nf"|format(...) on these fields, so those are the only files that can crash this way. The v1 (student_quiz_report.html) and v3 (student_quiz_report_v3.html) templates render the same values without format(...) and so cannot 500 on None.

Drive-by

app/routers/student_quiz_reports.py line 656 uses json.load(...) but json was never imported (introduced March 2025) — flake8 caught it when committing this change. Added import json at the top. Latent NameError on the v3 codepath if that line is ever hit.

Follow-up (separate ticket worth filing)

The v2 producer / ETL should emit 0.0 (or an explicit not_attempted: true flag) instead of DynamoDB NULL for accuracy when no questions were attempted. Existing rows with accuracy: NULL would also benefit from a one-shot backfill. I scoped this PR to a hotfix so the page stops 500-ing today; the data-quality cleanup is its own piece of work.

Test plan

• Local: hit the v2 template with a constructed report where subject_performance[*].accuracy = None and confirm it renders 0.0% instead of 500-ing.
• After deploy to staging: open the live URL /reports/student_quiz_report/PunjabStudents_69d5060a9be5a82f7e85209e/204716 and confirm the page renders. Maths row should show 0.0% accuracy.
• Open a known-good report (any student with all subjects attempted) and confirm the numbers are unchanged.
• PDF path: hit the same URL with ?format=pdf and confirm the print template also renders.

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