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/worker/init.go

@@ -0,0 +1,58 @@
+package main
+
+import (
+	"context"
+
+	"github.com/food-ai/backend/internal/adapters/kafka"
+	"github.com/food-ai/backend/internal/adapters/openai"
+	"github.com/food-ai/backend/internal/domain/dish"
+	"github.com/food-ai/backend/internal/domain/recognition"
+	"github.com/jackc/pgx/v5/pgxpool"
+	"github.com/kelseyhightower/envconfig"
+)
+
+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"`
+}
+
+func loadConfig() (*workerConfig, error) {
+	var workerCfg workerConfig
+	if configError := envconfig.Process("", &workerCfg); configError != nil {
+		return nil, configError
+	}
+	return &workerCfg, nil
+}
+
+// WorkerApp bundles background services that need lifecycle management.
+type WorkerApp struct {
+	workerPool *recognition.WorkerPool
+}
+
+// Start launches the worker pool goroutines.
+func (workerApp *WorkerApp) Start(applicationContext context.Context) {
+	workerApp.workerPool.Start(applicationContext)
+}
+
+func initWorker(workerCfg *workerConfig, pool *pgxpool.Pool) (*WorkerApp, error) {
+	openaiClient := openai.NewClient(workerCfg.OpenAIAPIKey)
+	dishRepository := dish.NewRepository(pool)
+	jobRepository := recognition.NewJobRepository(pool)
+
+	paidConsumer, paidConsumerError := kafka.NewConsumer(
+		workerCfg.KafkaBrokers, "dish-recognition-workers", recognition.TopicPaid,
+	)
+	if paidConsumerError != nil {
+		return nil, paidConsumerError
+	}
+	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)
+	return &WorkerApp{workerPool: workerPool}, nil
+}

backend/cmd/worker/main.go

@@ -0,0 +1,58 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"os"
+	"os/signal"
+	"syscall"
+
+	"github.com/food-ai/backend/internal/infra/database"
+	"github.com/food-ai/backend/internal/infra/locale"
+)
+
+func main() {
+	logger := slog.New(slog.NewJSONHandler(os.Stdout, &slog.HandlerOptions{
+		Level: slog.LevelInfo,
+	}))
+	slog.SetDefault(logger)
+
+	if runError := run(); runError != nil {
+		slog.Error("fatal error", "err", runError)
+		os.Exit(1)
+	}
+}
+
+func run() error {
+	workerConfig, configError := loadConfig()
+	if configError != nil {
+		return fmt.Errorf("load config: %w", configError)
+	}
+
+	applicationContext, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+
+	pool, poolError := database.NewPool(applicationContext, workerConfig.DatabaseURL)
+	if poolError != nil {
+		return fmt.Errorf("connect to database: %w", poolError)
+	}
+	defer pool.Close()
+	slog.Info("connected to database")
+
+	if loadError := locale.LoadFromDB(applicationContext, pool); loadError != nil {
+		return fmt.Errorf("load languages: %w", loadError)
+	}
+
+	workerApp, initError := initWorker(workerConfig, pool)
+	if initError != nil {
+		return fmt.Errorf("init worker: %w", initError)
+	}
+
+	workerApp.Start(applicationContext)
+	slog.Info("worker started")
+
+	<-applicationContext.Done()
+	slog.Info("worker shutting down...")
+	return nil
+}