[Improvement] Tracking: Add usr_id Index to ut_lp_marks

[fhelfer]

Performance Optimization: Add Single-Column Indexes for User Deletion Queries

The user deletion process (ilObjUser::delete()) exhibits performance issues, particularly when deleting users with extensive learning progress. Analysis of the deletion flow reveals that critical DELETE queries are executed without optimal index support.

1. ilObjUserTracking::_deleteUser() executes:

   DELETE FROM ut_lp_marks WHERE usr_id = ?

   The table ut_lp_marks has a composite PRIMARY KEY (obj_id, usr_id), but no single-column index on usr_id. Since usr_id is not the leading column in the primary key, MySQL cannot efficiently use this index for queries filtering only on usr_id, resulting in inefficient table scans or range scans over the clustered index.

Related to https://mantis.ilias.de/view.php?id=31664