Complete foundational data layer with repository implementation

- Add GORM dependencies for SQLite and PostgreSQL
- Create domain models (User, Session, File) with common errors
- Implement repository interfaces and database layer with migrations
- Update WebApp to bootstrap with database and repositories
- Add comprehensive unit tests for repository methods
- Update config structure to support multiple database drivers
- Extend AGENTS.md with debugging principles and dependency rules

internal/repository/file.go

@@ -0,0 +1,93 @@
+package repository
+
+import (
+	"context"
+	"errors"
+
+	"gorm.io/gorm"
+
+	"github.com/dhao2001/mygo/internal/model"
+)
+
+// FileRepository provides access to file records.
+type FileRepository interface {
+	Create(ctx context.Context, file *model.File) error
+	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)
+	Update(ctx context.Context, file *model.File) error
+	Delete(ctx context.Context, id string) error
+}
+
+type fileRepository struct {
+	db *gorm.DB
+}
+
+// NewFileRepository creates a FileRepository backed by GORM.
+func NewFileRepository(db *gorm.DB) FileRepository {
+	return &fileRepository{db: db}
+}
+
+func (r *fileRepository) Create(ctx context.Context, file *model.File) error {
+	result := r.db.WithContext(ctx).Create(file)
+	if result.Error != nil {
+		if isDuplicateKeyError(result.Error) {
+			return model.ErrDuplicate
+		}
+		return result.Error
+	}
+	return nil
+}
+
+func (r *fileRepository) FindByID(ctx context.Context, id string) (*model.File, error) {
+	var file model.File
+	result := r.db.WithContext(ctx).First(&file, "id = ?", id)
+	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) FindByUserID(ctx context.Context, userID 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 = ?", userID).Count(&total).Error; err != nil {
+		return nil, 0, err
+	}
+
+	result := r.db.WithContext(ctx).Where("user_id = ?", userID).Offset(offset).Limit(limit).Find(&files)
+	if result.Error != nil {
+		return nil, 0, result.Error
+	}
+
+	return files, total, nil
+}
+
+func (r *fileRepository) FindByParentID(ctx context.Context, userID string, parentID *string) ([]model.File, error) {
+	var files []model.File
+	result := r.db.WithContext(ctx).Where("user_id = ? AND parent_id IS ?", userID, parentID).Find(&files)
+	if result.Error != nil {
+		return nil, result.Error
+	}
+	return files, nil
+}
+
+func (r *fileRepository) Update(ctx context.Context, file *model.File) error {
+	result := r.db.WithContext(ctx).Save(file)
+	if result.Error != nil {
+		return result.Error
+	}
+	return nil
+}
+
+func (r *fileRepository) Delete(ctx context.Context, id string) error {
+	result := r.db.WithContext(ctx).Delete(&model.File{}, "id = ?", id)
+	if result.Error != nil {
+		return result.Error
+	}
+	return nil
+}