Approve Experiment

Approve a Draft experiment: set approved_at and create its FIRST run, atomically, in one request transaction.

Order: (1) reject with 400 if the Draft has no active prompt (before any write); (2) atomic compare-and-set approved_at (0 rows → already approved → 409); (3) create the first run for THIS experiment with on_active_conflict="raise" — a hotel active slot held by another experiment raises the active-run IntegrityError → 409 and the whole transaction (incl. approved_at) rolls back; (4) commit once, then dispatch the run in the background only when it was actually created.