feat: split worker into separate binary (cmd/worker)

Kafka consumers and WorkerPool are moved out of the server process into
a dedicated worker binary. Server now handles HTTP + SSE only; worker
handles Kafka consumption and AI processing.

- cmd/worker/main.go + init.go: new binary with minimal config
  (DATABASE_URL, OPENAI_API_KEY, KAFKA_BROKERS)
- cmd/server: remove WorkerPool, paidConsumer, freeConsumer
- Dockerfile: builds both server and worker binaries
- docker-compose.yml: add worker service
- Makefile: add run-worker and docker-logs-worker targets
- README.md: document worker startup and env vars

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

backend/cmd/server/init.go

@@ -46,24 +46,15 @@ func initApp(appConfig *config.Config, pool *pgxpool.Pool) (*App, error) {
 	ingredientHandler := ingredient.NewHandler(ingredientRepository)
 	productHandler := product.NewHandler(productRepository)
 
-	// Kafka producer and consumers
+	// Kafka producer
 	kafkaProducer, kafkaProducerError := newKafkaProducer(appConfig)
 	if kafkaProducerError != nil {
 		return nil, kafkaProducerError
 	}
-	paidConsumer, paidConsumerError := newPaidKafkaConsumer(appConfig)
-	if paidConsumerError != nil {
-		return nil, paidConsumerError
-	}
-	freeConsumer, freeConsumerError := newFreeKafkaConsumer(appConfig)
-	if freeConsumerError != nil {
-		return nil, freeConsumerError
-	}
 
 	// Recognition pipeline
 	jobRepository := recognition.NewJobRepository(pool)
 	sseBroker := recognition.NewSSEBroker(pool, jobRepository)
-	workerPool := recognition.NewWorkerPool(jobRepository, openaiClient, dishRepository, paidConsumer, freeConsumer)
 	recognitionHandler := recognition.NewHandler(openaiClient, ingredientRepository, jobRepository, kafkaProducer, sseBroker)
 
 	menuRepository := menu.NewRepository(pool)
@@ -101,8 +92,7 @@ func initApp(appConfig *config.Config, pool *pgxpool.Pool) (*App, error) {
 		mainTagListHandler,
 	)
 	return &App{
-		handler:    httpHandler,
-		workerPool: workerPool,
-		sseBroker:  sseBroker,
+		handler:   httpHandler,
+		sseBroker: sseBroker,
 	}, nil
 }