Per-queue picker loop (spec §6). Demand-driven: each poll picks at most the number of free concurrency slots and runs each instance in a supervised Task. Also heartbeats the leases of in-flight instances (one batched UPDATE per tick, regardless of concurrency).

partition_key serialization (spec §6) is handled per-job: a session-level advisory lock is taken on a checked-out connection held for the whole step, and released after the outcome commits. If the lock is contended, the row is returned to runnable and skipped.