Build a BullMQ job flow with parent-child dependencies using FlowProducer

Steps

1. Instantiate a FlowProducer connected to the same Redis instance as your queues
2. Call flowProducer.add() with a nested object describing the parent job and a 'children' array of child job definitions, each targeting its own queue name
3. BullMQ atomically creates the parent job plus all child jobs in one call; the parent enters the special 'waiting-children' state and will not move to 'waiting' (eligible for processing) until every child completes successfully
4. Run workers for each child queue as usual; once all children of a parent complete, the parent job automatically transitions to 'waiting' and is picked up by its own worker
5. In the parent job's processor, call job.getChildrenValues() to retrieve the return values of completed children keyed by job key, and aggregate/act on them
6. Nest flows arbitrarily deep by giving a child job its own 'children' array to build multi-level job hierarchies