Pre-register the email of an employee

Creates an enrollment request in pending status with a short expiry (ttl_minutes, 20 by default, between 1 and 60) for the employee to confirm by opening the mobile app and entering that email (POST /user-sync-validation/, no contract change for the app). It is idempotent: repeating the call for the same (api_key, email) while it is still pending and not expired returns the same request. If the email is already linked to the calling customer, it returns status "active" directly. If it is already linked to a different customer, it returns 409. Rate limit: at most 60 calls to this endpoint per api_key every 60 seconds, counting every attempt and not only the ones that create a row, checked before anything else so the answers that create nothing (200 already active, 409 linked to another customer, 400) cannot be walked as an enumeration oracle. Exceeding it returns 429 with code rate_limited.