refactor: split worker into paid/free via WORKER_PLAN env var

Replace dual-consumer priority WorkerPool with a single consumer per
worker process. WORKER_PLAN=paid|free selects the Kafka topic and
consumer group ID (dish-recognition-paid / dish-recognition-free).

docker-compose now runs worker-paid and worker-free as separate services
for independent scaling. Makefile dev target launches both workers locally.

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

backend/Makefile

@@ -1,4 +1,4 @@
-.PHONY: run run-worker dev dev-infra-up dev-infra-down test test-integration lint migrate-up migrate-down migrate-create migrate-status docker-up docker-down docker-logs docker-logs-worker
+.PHONY: run run-worker-paid run-worker-free dev dev-infra-up dev-infra-down test test-integration lint migrate-up migrate-down migrate-create migrate-status docker-up docker-down docker-logs docker-logs-worker
 
 ifneq (,$(wildcard .env))
   include .env
@@ -9,14 +9,18 @@ endif
 run:
 	go run ./cmd/server
 
-run-worker:
+run-worker-paid:
-	go run ./cmd/worker
+	WORKER_PLAN=paid go run ./cmd/worker
 
-# Start only infra (postgres, kafka) in Docker, run server + worker locally
+run-worker-free:
+	WORKER_PLAN=free go run ./cmd/worker
+
+# Start only infra (postgres, kafka) in Docker, run server + both workers locally
 dev: dev-infra-up
 	@trap 'kill 0' INT; \
 	go run ./cmd/server & \
-	go run ./cmd/worker & \
+	WORKER_PLAN=paid go run ./cmd/worker & \
+	WORKER_PLAN=free go run ./cmd/worker & \
 	wait
 
 dev-infra-up:
@@ -61,4 +65,4 @@ docker-logs:
 	docker compose logs -f app
 
 docker-logs-worker:
-	docker compose logs -f worker
+	docker compose logs -f worker-paid worker-free

backend/cmd/worker/init.go

@@ -15,6 +15,7 @@ type workerConfig struct {
 	DatabaseURL  string   `envconfig:"DATABASE_URL" required:"true"`
 	OpenAIAPIKey string   `envconfig:"OPENAI_API_KEY" required:"true"`
 	KafkaBrokers []string `envconfig:"KAFKA_BROKERS" default:"kafka:9092"`
+	WorkerPlan   string   `envconfig:"WORKER_PLAN" default:"free"` // "paid" | "free"
 }
 
 func loadConfig() (*workerConfig, error) {
@@ -40,19 +41,18 @@ func initWorker(workerCfg *workerConfig, pool *pgxpool.Pool) (*WorkerApp, error)
 	dishRepository := dish.NewRepository(pool)
 	jobRepository := recognition.NewJobRepository(pool)
 
-	paidConsumer, paidConsumerError := kafka.NewConsumer(
+	topic := recognition.TopicFree
-		workerCfg.KafkaBrokers, "dish-recognition-workers", recognition.TopicPaid,
+	groupID := "dish-recognition-free"
-	)
+	if workerCfg.WorkerPlan == "paid" {
-	if paidConsumerError != nil {
+		topic = recognition.TopicPaid
-		return nil, paidConsumerError
+		groupID = "dish-recognition-paid"
-	}
-	freeConsumer, freeConsumerError := kafka.NewConsumer(
-		workerCfg.KafkaBrokers, "dish-recognition-workers", recognition.TopicFree,
-	)
-	if freeConsumerError != nil {
-		return nil, freeConsumerError
 	}
 
-	workerPool := recognition.NewWorkerPool(jobRepository, openaiClient, dishRepository, paidConsumer, freeConsumer)
+	consumer, consumerError := kafka.NewConsumer(workerCfg.KafkaBrokers, groupID, topic)
+	if consumerError != nil {
+		return nil, consumerError
+	}
+
+	workerPool := recognition.NewWorkerPool(jobRepository, openaiClient, dishRepository, consumer)
 	return &WorkerApp{workerPool: workerPool}, nil
 }

backend/docker-compose.yml

@@ -69,7 +69,7 @@ services:
     volumes:
       - ./firebase-credentials.json:/app/firebase-credentials.json:ro
 
-  worker:
+  worker-paid:
     build:
       context: .
       dockerfile: Dockerfile
@@ -78,10 +78,25 @@ services:
       DATABASE_URL: postgres://food_ai:food_ai_local@postgres:5432/food_ai?sslmode=disable
       OPENAI_API_KEY: ${OPENAI_API_KEY}
       KAFKA_BROKERS: kafka:9092
+      WORKER_PLAN: paid
     depends_on:
       postgres:
         condition: service_healthy
-      kafka:
+      kafka-init:
+        condition: service_completed_successfully
+
+  worker-free:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    command: ["./worker"]
+    environment:
+      DATABASE_URL: postgres://food_ai:food_ai_local@postgres:5432/food_ai?sslmode=disable
+      OPENAI_API_KEY: ${OPENAI_API_KEY}
+      KAFKA_BROKERS: kafka:9092
+      WORKER_PLAN: free
+    depends_on:
+      postgres:
         condition: service_healthy
       kafka-init:
         condition: service_completed_successfully

backend/internal/domain/recognition/worker.go

@@ -4,50 +4,42 @@ import (
 	"context"
 	"log/slog"
 	"sync"
-	"time"
 
 	"github.com/food-ai/backend/internal/adapters/kafka"
 )
 
 const defaultWorkerCount = 5
 
-// WorkerPool processes dish recognition jobs from Kafka with priority queuing.
+// WorkerPool processes dish recognition jobs from a single Kafka topic.
-// Paid jobs are processed before free jobs.
 type WorkerPool struct {
 	jobRepo     JobRepository
 	recognizer  Recognizer
 	dishRepo    DishRepository
-	paidConsumer *kafka.Consumer
+	consumer    *kafka.Consumer
-	freeConsumer *kafka.Consumer
 	workerCount int
-	paidJobs     chan string
+	jobs        chan string
-	freeJobs     chan string
 }
 
-// NewWorkerPool creates a WorkerPool with five workers.
+// NewWorkerPool creates a WorkerPool with five workers consuming from a single consumer.
 func NewWorkerPool(
 	jobRepo JobRepository,
 	recognizer Recognizer,
 	dishRepo DishRepository,
-	paidConsumer *kafka.Consumer,
+	consumer *kafka.Consumer,
-	freeConsumer *kafka.Consumer,
 ) *WorkerPool {
 	return &WorkerPool{
 		jobRepo:     jobRepo,
 		recognizer:  recognizer,
 		dishRepo:    dishRepo,
-		paidConsumer: paidConsumer,
+		consumer:    consumer,
-		freeConsumer: freeConsumer,
 		workerCount: defaultWorkerCount,
-		paidJobs:     make(chan string, 100),
+		jobs:        make(chan string, 100),
-		freeJobs:     make(chan string, 100),
 	}
 }
 
-// Start launches the Kafka feeder goroutines and all worker goroutines.
+// Start launches the Kafka feeder goroutine and all worker goroutines.
 func (pool *WorkerPool) Start(workerContext context.Context) {
-	go pool.paidConsumer.Run(workerContext, pool.paidJobs)
+	go pool.consumer.Run(workerContext, pool.jobs)
-	go pool.freeConsumer.Run(workerContext, pool.freeJobs)
 	for i := 0; i < pool.workerCount; i++ {
 		go pool.runWorker(workerContext)
 	}
@@ -55,26 +47,11 @@ func (pool *WorkerPool) Start(workerContext context.Context) {
 
 func (pool *WorkerPool) runWorker(workerContext context.Context) {
 	for {
-		// Priority step: drain paid queue without blocking.
 		select {
-		case jobID := <-pool.paidJobs:
+		case jobID := <-pool.jobs:
-			pool.processJob(workerContext, jobID)
-			continue
-		case <-workerContext.Done():
-			return
-		default:
-		}
-
-		// Fall back to either queue with a 100ms timeout.
-		select {
-		case jobID := <-pool.paidJobs:
-			pool.processJob(workerContext, jobID)
-		case jobID := <-pool.freeJobs:
 			pool.processJob(workerContext, jobID)
 		case <-workerContext.Done():
 			return
-		case <-time.After(100 * time.Millisecond):
-			// nothing available; loop again
 		}
 	}
 }