Add file management API with local storage backend

- Implement FileHandler with CRUD operations for files/directories
- Add FileService with business logic and SHA-256 hashing
- Create LocalStorage backend for filesystem persistence
- Add database repository with pagination and name uniqueness
  constraints
- Configure max upload size in storage settings
- Include comprehensive tests for all layers

internal/repository/file.go

@@ -15,6 +15,8 @@ type FileRepository interface {
 	FindByID(ctx context.Context, id string) (*model.File, error)
 	FindByUserID(ctx context.Context, userID string, offset, limit int) ([]model.File, int64, error)
 	FindByParentID(ctx context.Context, userID string, parentID *string) ([]model.File, error)
+	FindByParentIDPaginated(ctx context.Context, userID string, parentID *string, offset, limit int) ([]model.File, int64, error)
+	FindByNameAndParent(ctx context.Context, userID string, parentID *string, name string) (*model.File, error)
 	Update(ctx context.Context, file *model.File) error
 	Delete(ctx context.Context, id string) error
 }
@@ -76,6 +78,34 @@ func (r *fileRepository) FindByParentID(ctx context.Context, userID string, pare
 	return files, nil
 }
 
+func (r *fileRepository) FindByParentIDPaginated(ctx context.Context, userID string, parentID *string, offset, limit int) ([]model.File, int64, error) {
+	var files []model.File
+	var total int64
+
+	if err := r.db.WithContext(ctx).Model(&model.File{}).Where("user_id = ? AND parent_id IS ?", userID, parentID).Count(&total).Error; err != nil {
+		return nil, 0, err
+	}
+
+	result := r.db.WithContext(ctx).Where("user_id = ? AND parent_id IS ?", userID, parentID).Offset(offset).Limit(limit).Order("is_dir DESC, name ASC").Find(&files)
+	if result.Error != nil {
+		return nil, 0, result.Error
+	}
+
+	return files, total, nil
+}
+
+func (r *fileRepository) FindByNameAndParent(ctx context.Context, userID string, parentID *string, name string) (*model.File, error) {
+	var file model.File
+	result := r.db.WithContext(ctx).Where("user_id = ? AND parent_id IS ? AND name = ?", userID, parentID, name).First(&file)
+	if errors.Is(result.Error, gorm.ErrRecordNotFound) {
+		return nil, model.ErrNotFound
+	}
+	if result.Error != nil {
+		return nil, result.Error
+	}
+	return &file, nil
+}
+
 func (r *fileRepository) Update(ctx context.Context, file *model.File) error {
 	result := r.db.WithContext(ctx).Save(file)
 	if result.Error != nil {