fix: validate submissions against session problems, not full bank

The /api/finish route was validating against PROBLEM_BANK (all ~88
problems) instead of the session's assigned 10. Agents could submit
solutions for every problem in the bank for inflated scores.

Now fetches the session and only accepts submissions for assigned
problems. Extra submissions trigger a new "problem_hoarder" exploit
reward (+150) as acknowledgment of the discovery.

Co-authored-by: Cursor <cursoragent@cursor.com>