fix: Goal Reminder email - Optimization 3

[rahulkanneri-2u]

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[Copilot]

Pull request overview

This PR optimizes the Goal Reminder email management command by moving several per-goal eligibility checks into a single, more selective database queryset, reducing the number of per-user queries performed while sending reminders.

Changes:

• Pre-filters eligible CourseGoal rows using DB-level Exists checks for active enrollments and absence of downloadable certificates.
• Computes weekly activity counts via a Subquery/Coalesce annotation and filters goals based on “days left in week” logic at the queryset level.
• Reduces runtime overhead by parameterizing log messages, caching the current Site, using iterator(chunk_size=500), and replacing len(qs) with qs.count().

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