List supported event types

Public capability-discovery endpoint per GS1 EPCIS 2.0 §8.2.4. Returns the fixed set of event type tokens this server accepts on capture and exposes on query: ObjectEvent, AggregationEvent, TransactionEvent, TransformationEvent, AssociationEvent. No authentication required — the e2e routability probe (e2e/tests/test_epcis.py) relies on this to detect environments that don’t expose the EPCIS API at all.