Replace inefficient per-course document query with database aggregation
using Max() annotation. This reduces sitemap generation from N+1 queries
to just 1 query, where N is the number of courses.

Before:
- items() prefetched document_set for each course
- lastmod() called .order_by().first() for each course (N queries)
- For 1000 courses: 1001 queries

After:
- items() annotates with Max('document__created')
- lastmod() returns annotated field
- For 1000 courses: 1 query

This fix prevents database overload during sitemap generation and
significantly improves response time for sitemap endpoints.