Enqueue scheduled discovery targets

Enqueue a durable discovery job for each enabled schedule target and return immediately. Designed for invocation by Cloud Scheduler or other cron triggers; the durable worker performs the discovery work. Re-triggering the same target on the same day reuses the already-enqueued job, so a duplicate cron fire is a no-op.

Use this discovery operation to create, synchronize, lease, inspect, or cancel research work. Treat discovered actors as reviewable leads until their source evidence and visibility state are checked.