One broadcaster process per {schema, pubsub, topic} triple. Holds a single DustEcto.Repo.subscribe/2 registration whose callback broadcasts every event through Phoenix.PubSub on topic.

Lifecycle:

• Started lazily by DustEcto.Phoenix.subscribe_to_pubsub/3.
• Registered in DustEcto.Phoenix.Registry keyed by {schema, pubsub, topic} so concurrent callers reuse the same broadcaster.
• Stays alive for the application lifetime. The cost is one process and one entry in the SDK callback registry — both negligible. Cleanup is opt-in (caller can unsubscribe/3).

The broadcast message shape is {:dust_event, event} where event is whatever DustEcto.Repo.subscribe/2 would deliver: {:upserted, struct} or {:deleted, slug}.