Cache leaderboard data to improve page load performance

[Copilot]

Leaderboard pages experienced slow loading due to repeated database queries fetching all course enrollments on each request.

Changes

Cache configuration

• Added LocMemCache backend in Django settings for in-memory caching

Leaderboard view (courses/views/course.py)

• Cache enrollment data as dictionaries (not model instances to avoid stale field values)
• Cache key: leaderboard:{course_id}, TTL: 1 hour
• Example:

cache_key = f"leaderboard:{course.id}"
enrollments_data = cache.get(cache_key)

if enrollments_data is None:
    enrollments = Enrollment.objects.filter(course=course).select_related('student').order_by(...)
    enrollments_data = [
        {'id': e.id, 'display_name': e.display_name, 'total_score': e.total_score, ...}
        for e in enrollments
    ]
    cache.set(cache_key, enrollments_data, 3600)

Cache invalidation (courses/scoring.py)

• update_leaderboard() deletes cache after bulk updating enrollment scores
• Triggered automatically on homework/project scoring completion

Test updates

• Added cache invalidation test
• Clear cache in test setUp to prevent cross-test pollution
• Updated assertions to work with dictionary data

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• astral.sh
  • Triggering command: /usr/bin/curl curl -LsSf REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Page slow loading time</issue_title>
<issue_description>Some pages show very slow loading time, especially leaderboard.

For leaderboard let's cache it and invalidate the cache when some important events happen (leaderboard recalculation)

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Page slow loading time #147

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