Option to prevent overlapping runs for a check

Sometimes a check will run concurrently, like if the check has a scheduled run that overlaps with CI/CD- or locally-triggered runs.

This is an issue for certain checks, especially login flows where the server only allows one login session per account. When the same check runs concurrently, they interfere with each other (like the first check logging out of the session, causing the second check to fail).

It would help to have a per-check toggle (max concurrency = 1) that waits for existing runs to finish if the same check is already in-flight, regardless of trigger source.