Ergon's root supervisor.

Child order is the dependency order, and every step of it matters:

ergon_sql                  every consumer runs named statements through it
ergon_listener             the node's shared LISTEN connection
ergon_worker_registry      the pg scope; workers join it in their own init/1
ergon_job_notifier         optional; dispatches wakes through the registry
ergon_worker_sup           dynamic; queue workers appear under it at runtime
ergon_pgmq_consumer_sup    dynamic; pgmq consumers likewise

one_for_one, so a notifier crash does not disturb running workers. The notifier is the only optional child: without it workers still drain on their fallback poll, correctly but with more latency.

ergon_listener precedes both the notifier and the consumer supervisor, because both subscribe to it from their own init/1. It is started unconditionally even when the job notifier is disabled, since a pgmq consumer may still want it.

The connection pool is not a child. ergon_app:start/2 starts it before this tree and pgo_sup supervises it; see ergon_repo for why that is not turned into a child spec.