Add Kafka pub/sub backend

Third backend alongside Pulsar and RabbitMQ. Topics map 1:1 to Kafka
topics, subscriptions map to consumer groups. Response/notify uses
unique consumer groups with correlation ID filtering. Topic lifecycle
managed via AdminClient with class-based retention.

trustgraph-base/pyproject.toml

@@ -15,6 +15,7 @@ dependencies = [
     "requests",
     "python-logging-loki",
     "pika",
+    "confluent-kafka",
     "pyyaml",
 ]
 classifiers = [