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

internal/service/logger.go

@@ -2,10 +2,10 @@ package service
 
 import (
 	"context"
-	"log"
 	"time"
 
 	"github.com/ez-api/ez-api/internal/model"
+	"github.com/rs/zerolog/log"
 	"gorm.io/gorm"
 )
 
@@ -47,7 +47,7 @@ func (w *LogWriter) Start(ctx context.Context) {
 				return
 			}
 			if err := w.db.Create(&buf).Error; err != nil {
-				log.Printf("log batch insert failed: %v", err)
+				log.Error().Err(err).Msg("log batch insert failed")
 			}
 			buf = buf[:0]
 		}