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
+}