Set Schema Registry compatibility mode per subject and evolve an Avro schema safely

Steps

1. Check the global default compatibility mode: GET /config returns {"compatibilityLevel": "BACKWARD"} — Confluent Schema Registry defaults to BACKWARD
2. Override compatibility for a specific subject: PUT /config/my-topic-value with body {"compatibility": "FULL_TRANSITIVE"} to require every new version to be both backward and forward compatible with all prior versions
3. Test a candidate schema before registering: POST /compatibility/subjects/my-topic-value/versions/latest with the new schema; a 200 response with {"is_compatible": true} confirms it passes
4. Register the new schema version: POST /subjects/my-topic-value/versions with the schema JSON
5. For BACKWARD mode: only add fields with defaults and only delete fields without defaults; upgrade all consumers before producers
6. For FORWARD mode: only add optional fields (fields with defaults) and delete fields that already have defaults; upgrade producers first, then consumers