Publish RabbitMQ messages as JSON instead of Java serialization

The avatar update event (and any other RabbitTemplate publish) was serialized
with the default SimpleMessageConverter, producing
application/x-java-serialized-object instead of JSON. The JacksonJsonMessageConverter
was only wired into the listener container factory (consuming), not the
auto-configured RabbitTemplate (publishing).

Expose a single JacksonJsonMessageConverter bean so Spring Boot applies it to the
auto-configured RabbitTemplate, and reuse it in the listener factory.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Brutus5000

src/main/java/com/faforever/api/config/RabbitConfiguration.java

@@ -55,16 +55,29 @@ public StatelessRetryOperationsInterceptor retryInterceptor() {
    * Reconfigure default Rabbit container, so it doesn't infinitely requeue.
    * Instead, we use a retry that does a limited requeueing.
    */
+  /**
+   * JSON message converter used for both publishing (applied to the auto-configured
+   * {@link org.springframework.amqp.rabbit.core.RabbitTemplate} by Spring Boot when a single
+   * {@link org.springframework.amqp.support.converter.MessageConverter} bean is present) and consuming.
+   * Without this, the default {@code SimpleMessageConverter} would Java-serialize outgoing payloads
+   * ({@code application/x-java-serialized-object}) instead of producing JSON.
+   */
+  @Bean
+  public JacksonJsonMessageConverter jacksonJsonMessageConverter() {
+    return new JacksonJsonMessageConverter();
+  }
+
   @Bean
   public SimpleRabbitListenerContainerFactory rabbitListenerContainerFactory(
     ConnectionFactory connectionFactory,
-    StatelessRetryOperationsInterceptor retryInterceptor
+    StatelessRetryOperationsInterceptor retryInterceptor,
+    JacksonJsonMessageConverter jacksonJsonMessageConverter
   ) {
     var factory = new SimpleRabbitListenerContainerFactory();
     factory.setDefaultRequeueRejected(false);
     factory.setConnectionFactory(connectionFactory);
     factory.setAdviceChain(retryInterceptor);
-    factory.setMessageConverter(new JacksonJsonMessageConverter());
+    factory.setMessageConverter(jacksonJsonMessageConverter);
 
     return factory;
   }