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/adapters/postgres/Cargo.toml

@@ -5,7 +5,7 @@ edition = "2024"
 
 [dependencies]
 domain        = { workspace = true }
-event-payload = { workspace = true }
+adapters-event-payload = { workspace = true }
 sqlx          = { workspace = true, features = ["postgres", "runtime-tokio", "migrate", "uuid", "chrono", "json"] }
 uuid          = { workspace = true }
 chrono        = { workspace = true }