Add 4-Year Expiration Date to Student Background Checks

Issue Description: Each student background check should have an expiration date. Background checks are currently stored individually for each student, but there is no expiration period associated with them. Add an expiration date based on a 4-year validity period from the applicable background check date.

Expected Behavior: A student's background check should remain valid for four years. The system should track the expiration date for each background check and identify when a background check has expired so a new one can be completed.

No files or tests are named. Start by locating the student background-check model and the workflow that creates or displays checks; trace which date is the applicable check date. Done means each check has a four-year expiration date and the system identifies expired checks so a replacement can be completed.

Written by the indexing model from the issue text.