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/service/file_test.go

@@ -3,7 +3,9 @@ package service
 import (
 	"bytes"
 	"context"
+	"errors"
 	"io"
+	"net/http"
 	"strings"
 	"sync"
 	"testing"
@@ -16,6 +18,21 @@ import (
 	"github.com/dhao2001/mygo/internal/storage"
 )
 
+// assertAppError checks that err is an *model.AppError with the expected status.
+func assertAppError(t *testing.T, err error, wantStatus int) bool {
+	t.Helper()
+	var ae *model.AppError
+	if !errors.As(err, &ae) {
+		t.Errorf("expected *model.AppError, got %T: %v", err, err)
+		return false
+	}
+	if ae.Status != wantStatus {
+		t.Errorf("status = %d, want %d; message = %q", ae.Status, wantStatus, ae.Message)
+		return false
+	}
+	return true
+}
+
 // memStorage is an in-memory implementation of storage.StorageBackend for tests.
 type memStorage struct {
 	mu    sync.RWMutex
@@ -70,7 +87,7 @@ func setupFileService(t *testing.T) *FileService {
 	fileRepo := repository.NewFileRepository(db)
 	store := newMemStorage()
 
-	return NewFileService(fileRepo, store, 0) // 0 = unlimited
+	return NewFileService(fileRepo, store, 0, nil) // 0 = unlimited, nil logger defaults to slog.Default()
 }
 
 func TestFileService_Upload(t *testing.T) {
@@ -214,9 +231,7 @@ func TestFileService_DownloadForbidden(t *testing.T) {
 	}
 
 	_, _, err = svc.Download(ctx, "user2", info.ID)
-	if err != model.ErrForbidden {
-		t.Errorf("expected ErrForbidden, got %v", err)
-	}
+	assertAppError(t, err, http.StatusForbidden)
 }
 
 func TestFileService_DownloadDirectory(t *testing.T) {
@@ -257,9 +272,7 @@ func TestFileService_GetNotFound(t *testing.T) {
 	ctx := context.Background()
 
 	_, err := svc.Get(ctx, "user1", "nonexistent")
-	if err != model.ErrNotFound {
-		t.Errorf("expected ErrNotFound, got %v", err)
-	}
+	assertAppError(t, err, http.StatusNotFound)
 }
 
 func TestFileService_GetForbidden(t *testing.T) {
@@ -272,9 +285,7 @@ func TestFileService_GetForbidden(t *testing.T) {
 	}
 
 	_, err = svc.Get(ctx, "user2", info.ID)
-	if err != model.ErrForbidden {
-		t.Errorf("expected ErrForbidden, got %v", err)
-	}
+	assertAppError(t, err, http.StatusForbidden)
 }
 
 func TestFileService_List(t *testing.T) {
@@ -372,9 +383,7 @@ func TestFileService_Delete(t *testing.T) {
 	}
 
 	_, err = svc.Get(ctx, "user1", info.ID)
-	if err != model.ErrNotFound {
-		t.Errorf("expected ErrNotFound after delete, got %v", err)
-	}
+	assertAppError(t, err, http.StatusNotFound)
 }
 
 func TestFileService_DeleteNonEmptyDirectory(t *testing.T) {
@@ -406,9 +415,7 @@ func TestFileService_DeleteForbidden(t *testing.T) {
 	}
 
 	err = svc.Delete(ctx, "user2", info.ID)
-	if err != model.ErrForbidden {
-		t.Errorf("expected ErrForbidden, got %v", err)
-	}
+	assertAppError(t, err, http.StatusForbidden)
 }
 
 func TestFileService_CreateDir(t *testing.T) {
@@ -452,7 +459,5 @@ func TestFileService_VerifyParentForbidden(t *testing.T) {
 	}
 
 	_, err = svc.Upload(ctx, "user2", &dir.ID, "stolen.txt", strings.NewReader("data"))
-	if err != model.ErrForbidden {
-		t.Errorf("expected ErrForbidden, got %v", err)
-	}
+	assertAppError(t, err, http.StatusForbidden)
 }