feat: add dev make target — infra in Docker, server+worker run locally

- Add EXTERNAL listener on port 9093 to Kafka so local processes can connect
- Add KAFKA_BROKERS=localhost:9093 to .env.example
- Add dev/dev-infra-up/dev-infra-down targets to Makefile

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/.env.example

@@ -13,6 +13,9 @@ JWT_REFRESH_DURATION=720h
 PORT=8080
 ALLOWED_ORIGINS=http://localhost:3000
 
+# Kafka (localhost:9093 for local dev, kafka:9092 when running inside Docker)
+KAFKA_BROKERS=localhost:9093
+
 # External APIs
 OPENAI_API_KEY=your-openai-key
 PEXELS_API_KEY=your-pexels-key