Add createdBefore Filter to Candidate and Application List Endpoints

candidate.list and application.list now accept an optional createdBefore parameter (Unix timestamp in milliseconds) that returns only records created at or before the specified time, complementing the existing createdAfter filter. Together, these parameters enable bounded date-window queries for large historical backfills.
Both endpoints now return invalid_input if createdAfter is later than createdBefore, or if either parameter is set to a time in the future.