fix: pin a Kafka version the operator supports, and add the API's producer

Strimzi 1.2.0 supports 4.2.0 through 4.3.1; the manifest still asked for 4.1.0,
so the cluster sat NotReady with the reason only visible in the Kafka CR's
status — the operator logs said nothing. Pinned to 4.3.1, and metadataVersion
removed rather than guessed: the operator derives it, and a stale spelling there
is the same failure one release later.

Adds a KafkaUser for classic-models-api: Write and Describe on the
classic-models. prefix, and no Read. It publishes domain events and consumes
nothing, and a producer that can read every topic exfiltrates every other
domain's events if it is ever compromised.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -9,7 +9,7 @@ Single-broker Kafka cluster for dev/test, deployed via the [Strimzi](https://str
 ```
 kafka namespace
 ├── strimzi-cluster-operator   Strimzi operator — reconciles Kafka/KafkaUser/KafkaTopic CRs
-├── kafka-dual-role-0          Kafka 4.1.0 broker (KRaft, controller + broker in one pod)
+├── kafka-dual-role-0          Kafka 4.3.1 broker (KRaft, controller + broker in one pod)
 └── kafka-entity-operator      Topic Operator + User Operator (manages topics and users)
 ```
 
@@ -40,6 +40,12 @@ kafka-kafka-bootstrap.kafka.svc.cluster.local:9093
 |---|---|---|
 | `kafka-admin` | `kafka-admin` | Super user — unrestricted access |
 | `kafka-client` | `kafka-client` | Application user — Read/Write all topics and groups |
+| `classic-models-api` | `classic-models-api` | Producer for `classic-models.*` — **Write and Describe only**, no Read |
+
+`classic-models-api` publishes domain events and consumes nothing, so it is
+write-only and prefix-scoped. A producer that can also read every topic is a
+producer that exfiltrates every other domain's events if it is ever
+compromised.
 
 Strimzi stores credentials in Kubernetes secrets. Each secret contains two keys:
 - `password` — the SCRAM password