Add structured logging and centralized error handling

- Initialize slog in the serve command with terminal/file support
- Introduce `AppError` with HTTP status for unified service-layer errors
- Replace ad-hoc `api.Error` calls with `api.RespondError`
- Wrap internal errors with `model.NewInternalError` and add reference
  IDs
- Add `RequestID` middleware and switch router from `gin.Default` to
  `gin.New`

internal/config/load.go

@@ -27,6 +27,10 @@ func defaults(v *viper.Viper) {
 	v.SetDefault("jwt.secret", "dev-secret-do-not-use-in-production")
 	v.SetDefault("jwt.access_ttl", "15m")
 	v.SetDefault("jwt.refresh_ttl", "168h")
+
+	v.SetDefault("log.level", "info")
+	v.SetDefault("log.file_path", "")
+	v.SetDefault("log.file_level", "debug")
 }
 
 func New() *viper.Viper {