Consume CourseCreated from Kafka and remove old consumer infrastructure

[maalayat]

Parent

#1 — Migrate domain event bus from RabbitMQ to Kafka

What to build

Messages on the "course.created" Kafka topic are consumed, deserialized, and delivered to the IncrementCoursesCounterOnCourseCreated application service, completing the end-to-end event flow.

The old RabbitMQ consumer and reflection-based subscriber discovery machinery are removed. The application service is cleaned up to have no messaging annotations.

Acceptance criteria

• CourseCreatedKafkaConsumer created with @KafkaListener(topics = "course.created")
• IncrementCoursesCounterOnCourseCreated has no @EventListener annotation
• RabbitMQ consumer and reflection-based event infrastructure deleted
• spring-rabbit-test and org.testcontainers:rabbitmq removed from build.gradle
• End-to-end integration test: CourseCreator.create() → CoursesCounter incremented

Blocked by

• Switch primary event bus to Kafka and remove RabbitMQ producer #5 — Switch primary event bus to Kafka and remove RabbitMQ producer