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/app.go

@@ -9,9 +9,8 @@ import (
 
 // App bundles the HTTP handler with background services that need lifecycle management.
 type App struct {
-	handler    http.Handler
-	workerPool *recognition.WorkerPool
-	sseBroker  *recognition.SSEBroker
+	handler   http.Handler
+	sseBroker *recognition.SSEBroker
 }
 
 // ServeHTTP implements http.Handler.
@@ -19,9 +18,8 @@ func (application *App) ServeHTTP(responseWriter http.ResponseWriter, request *h
 	application.handler.ServeHTTP(responseWriter, request)
 }
 
-// Start launches the SSE broker's LISTEN loop and the worker pool goroutines.
+// Start launches the SSE broker's LISTEN loop.
 // Call this once before the HTTP server begins accepting connections.
 func (application *App) Start(applicationContext context.Context) {
 	application.sseBroker.Start(applicationContext)
-	application.workerPool.Start(applicationContext)
 }

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
 }

backend/cmd/server/providers.go

@@ -197,14 +197,6 @@ func newKafkaProducer(appConfig *config.Config) (*kafka.Producer, error) {
 	return kafka.NewProducer(appConfig.KafkaBrokers)
 }
 
-func newPaidKafkaConsumer(appConfig *config.Config) (*kafka.Consumer, error) {
-	return kafka.NewConsumer(appConfig.KafkaBrokers, "dish-recognition-workers", recognition.TopicPaid)
-}
-
-func newFreeKafkaConsumer(appConfig *config.Config) (*kafka.Consumer, error) {
-	return kafka.NewConsumer(appConfig.KafkaBrokers, "dish-recognition-workers", recognition.TopicFree)
-}
-
 // ---------------------------------------------------------------------------
 // Interface assertions (compile-time checks)
 // ---------------------------------------------------------------------------