Emitter behaviour for evoq.

Emitters subscribe to domain events and publish them as integration facts to external transports (pg or mesh). They translate domain events into facts using an evoq_fact module and publish to the target transport.

Emitters live in the source desk -- the desk that processes the command producing the event. They are supervised by the desk supervisor.

Naming convention: emit_{event}_to_{transport}

Required Callbacks

- source_event() -> atom() - fact_module() -> module() - transport() -> pg | mesh - emit(FactType, Payload, Metadata) -> ok | {error, Reason}