Event handler behavior for evoq.

Event handlers subscribe to event types (NOT streams) and process events as they are published. This is the key scalability improvement over per-stream subscriptions.

Callbacks

Required: - interested_in() -> [binary()] Returns list of event types this handler processes

- init(Config) -> {ok, State} Initialize handler state

- handle_event(EventType, Event, Metadata, State) -> {ok, NewState} | {error, Reason} Process a single event

Optional: - on_error(Error, Event, FailureContext, State) -> error_action() Handle errors during event processing

- replay_policy() -> skip | deliver What to do with REPLAY: events this node had already consumed before it restarted, which evoq_store_subscription hands out again on every boot with replaying => true in the metadata. skip for a handler with side effects (publishing, sending, dispatching), which would otherwise repeat every one of them on every restart. deliver for a handler rebuilding in-memory state from history.

Not declaring it delivers, as before this callback existed, and logs one warning per boot naming the handler the first time it is handed replay: a handler with side effects and no policy is the one to find.

- handle_info(Info, State) -> {noreply, NewState} | {stop, Reason, NewState} Any other message the handler process receives, most usefully one it scheduled to itself from handle_event/4 (a retry via send_after). Same shape as gen_server's. A handler without it that is sent a message logs a warning naming itself and the message.