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/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
+}