#784 HIGH: Event subscription silently overwrites existing subscription

create_event_subscription INSERT uses ON CONFLICT DO UPDATE, silently replacing existing subscription. If workflow A subscribes to event X, then workflow B subscribes to same event before A receives it, A's subscription is deleted. Location: migrations/01KCFHDFDPZ4MJM8H87RM56BVW:435-458. Fix: Use ON CONFLICT DO NOTHING.