fix: filter learner search by requested usernames instead of scanning all users

[Alam-2U]

Description

The _get_stats_for_usernames function in forum/api/users.py calls backend.get_users() with no arguments, triggering a full collection scan on the users collection for every activity_stats request.

This means viewing the Learners tab for a single course — or searching for a single learner — causes the database to iterate over millions of user documents across all courses, only to discard them via Python-side filtering.

Impact

• 500 errors on the Learners tab — the full scan hits a legacy document containing a corrupt byte (0xff at position 24) that PyMongo's UTF-8 decoder cannot handle, raising InvalidBSON
• Severe performance degradation — every Learners tab page load or search queries millions of documents instead of the 1–25 actually needed

Fix

Pass the usernames filter to backend.get_users() so the filtering happens at the database query level instead of in Python.

Ticket

LP-956

[Copilot]

Pull request overview

This PR optimizes the learner stats lookup path by pushing username filtering down into the database query (MongoDB/MySQL) instead of fetching all users and filtering in Python, preventing expensive full-collection scans and related failures.

Changes:

• Updated _get_stats_for_usernames to call backend.get_users(username__in=...) and removed Python-side filtering.
• Added username__in support to both MongoDB and MySQL backend get_users() implementations by translating it to the respective query syntax.
• Bumped the package version from 0.6.8 to 0.6.9.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
forum/backends/mysql/api.py	Maps username__in to Django ORM user__username__in for DB-level filtering.
forum/backends/mongodb/api.py	Maps username__in to MongoDB { username: { $in: [...] } } filtering.
forum/api/users.py	Uses backend filtering for username-specific stats requests and removes Python filtering.
forum/init.py	Version bump to reflect the change set.

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