refactor: code smell fixes — tests, events, naming

- Tests for ExecutePipelineHandler (happy path, fallback, disabled skip, failure retry, not found)
- Tests for ProcessNextJobHandler (empty queue, process, drain multiple)
- DerivativeGenerated domain event + event-payload mapping + event_store aggregate
- Renamed event-payload → adapters-event-payload, event-transport → adapters-event-transport

crates/domain/src/common/events.rs

@@ -39,6 +39,12 @@ pub enum DomainEvent {
         asset_id: SystemId,
         timestamp: DateTimeStamp,
     },
+    DerivativeGenerated {
+        asset_id: SystemId,
+        derivative_id: SystemId,
+        profile: String,
+        timestamp: DateTimeStamp,
+    },
     JobEnqueued {
         job_id: SystemId,
         job_type: String,