feat(log): replace standard logger with zerolog

- Integrate `rs/zerolog` for structured and leveled logging
- Add `EZ_LOG_LEVEL` environment variable support (default: info)
- Configure console output with timestamps and service fields
- Migrate `main.go` and `LogWriter` to use the new logger instance
- Update README with logging configuration and design details

cmd/server/main.go

@@ -2,10 +2,10 @@ package main
 
 import (
 	"context"
-	"log"
 	"net/http"
 	"os"
 	"os/signal"
+	"strings"
 	"syscall"
 	"time"
 
@@ -17,6 +17,8 @@ import (
 	"github.com/ez-api/ez-api/internal/service"
 	"github.com/gin-gonic/gin"
 	"github.com/redis/go-redis/v9"
+	"github.com/rs/zerolog"
+	"github.com/rs/zerolog/log"
 	swaggerFiles "github.com/swaggo/files"
 	ginSwagger "github.com/swaggo/gin-swagger"
 	"gorm.io/driver/postgres"
@@ -47,10 +49,12 @@ import (
 // @name Authorization
 
 func main() {
+	logger := setupLogger()
+
 	// 1. Load Configuration
 	cfg, err := config.Load()
 	if err != nil {
-		log.Fatalf("Failed to load config: %v", err)
+		logger.Fatal().Err(err).Msg("Failed to load config")
 	}
 
 	// 2. Initialize Redis Client
@@ -64,28 +68,28 @@ func main() {
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
 	if err := rdb.Ping(ctx).Err(); err != nil {
-		log.Fatalf("Failed to connect to Redis: %v", err)
+		logger.Fatal().Err(err).Msg("Failed to connect to Redis")
 	}
-	log.Println("Connected to Redis successfully")
+	logger.Info().Msg("Connected to Redis successfully")
 
 	// 3. Initialize GORM (PostgreSQL)
 	db, err := gorm.Open(postgres.Open(cfg.Postgres.DSN), &gorm.Config{})
 	if err != nil {
-		log.Fatalf("Failed to connect to PostgreSQL: %v", err)
+		logger.Fatal().Err(err).Msg("Failed to connect to PostgreSQL")
 	}
 	sqlDB, err := db.DB()
 	if err != nil {
-		log.Fatalf("Failed to get generic database object: %v", err)
+		logger.Fatal().Err(err).Msg("Failed to get generic database object")
 	}
 	// Verify DB connection
 	if err := sqlDB.Ping(); err != nil {
-		log.Fatalf("Failed to ping PostgreSQL: %v", err)
+		logger.Fatal().Err(err).Msg("Failed to ping PostgreSQL")
 	}
-	log.Println("Connected to PostgreSQL successfully")
+	logger.Info().Msg("Connected to PostgreSQL successfully")
 
 	// Auto Migrate
 	if err := db.AutoMigrate(&model.Master{}, &model.Key{}, &model.Provider{}, &model.Model{}, &model.LogRecord{}); err != nil {
-		log.Fatalf("Failed to auto migrate: %v", err)
+		logger.Fatal().Err(err).Msg("Failed to auto migrate")
 	}
 
 	// 4. Setup Services and Handlers
@@ -97,7 +101,7 @@ func main() {
 
 	adminService, err := service.NewAdminService()
 	if err != nil {
-		log.Fatalf("Failed to create admin service: %v", err)
+		logger.Fatal().Err(err).Msg("Failed to create admin service")
 	}
 	masterService := service.NewMasterService(db)
 	healthService := service.NewHealthCheckService(db, rdb)
@@ -108,7 +112,7 @@ func main() {
 
 	// 4.1 Prime Redis snapshots so DP can start with data
 	if err := syncService.SyncAll(db); err != nil {
-		log.Printf("Initial sync warning: %v", err)
+		logger.Warn().Err(err).Msg("Initial sync warning")
 	}
 
 	// 5. Setup Gin Router
@@ -172,9 +176,9 @@ func main() {
 
 	// 6. Start Server with Graceful Shutdown
 	go func() {
-		log.Printf("Starting ez-api on port %s", cfg.Server.Port)
+		logger.Info().Str("port", cfg.Server.Port).Msg("Starting ez-api")
 		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
-			log.Fatalf("Server failed: %v", err)
+			logger.Fatal().Err(err).Msg("Server failed")
 		}
 	}()
 
@@ -182,14 +186,36 @@ func main() {
 	quit := make(chan os.Signal, 1)
 	signal.Notify(quit, syscall.SIGINT, syscall.SIGTERM)
 	<-quit
-	log.Println("Shutting down server...")
+	logger.Info().Msg("Shutting down server...")
 
 	// Shutdown with timeout
 	ctx, cancel = context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
 	if err := srv.Shutdown(ctx); err != nil {
-		log.Fatalf("Server forced to shutdown: %v", err)
+		logger.Fatal().Err(err).Msg("Server forced to shutdown")
 	}
 
-	log.Println("Server exited properly")
+	logger.Info().Msg("Server exited properly")
+}
+
+func setupLogger() zerolog.Logger {
+	level := zerolog.InfoLevel
+	if raw := strings.TrimSpace(os.Getenv("EZ_LOG_LEVEL")); raw != "" {
+		if parsed, err := zerolog.ParseLevel(strings.ToLower(raw)); err == nil {
+			level = parsed
+		}
+	}
+
+	zerolog.SetGlobalLevel(level)
+	output := zerolog.ConsoleWriter{
+		Out:        os.Stdout,
+		TimeFormat: time.RFC3339,
+	}
+
+	logger := zerolog.New(output).Level(level).With().
+		Timestamp().
+		Str("service", "ez-api").
+		Logger()
+	log.Logger = logger
+	return logger
 }