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/user.go

@@ -0,0 +1,118 @@
+package repository
+
+import (
+	"context"
+	"errors"
+	"strings"
+
+	"gorm.io/gorm"
+
+	"github.com/dhao2001/mygo/internal/model"
+)
+
+// isDuplicateKeyError checks if the error indicates a unique constraint violation.
+func isDuplicateKeyError(err error) bool {
+	return errors.Is(err, gorm.ErrDuplicatedKey) || strings.Contains(strings.ToLower(err.Error()), "unique constraint failed")
+}
+
+// UserRepository provides access to user records.
+type UserRepository interface {
+	Create(ctx context.Context, user *model.User) error
+	FindByID(ctx context.Context, id string) (*model.User, error)
+	FindByEmail(ctx context.Context, email string) (*model.User, error)
+	FindByUsername(ctx context.Context, username string) (*model.User, error)
+	Update(ctx context.Context, user *model.User) error
+	Delete(ctx context.Context, id string) error
+	List(ctx context.Context, offset, limit int) ([]model.User, int64, error)
+}
+
+type userRepository struct {
+	db *gorm.DB
+}
+
+// NewUserRepository creates a UserRepository backed by GORM.
+func NewUserRepository(db *gorm.DB) UserRepository {
+	return &userRepository{db: db}
+}
+
+func (r *userRepository) Create(ctx context.Context, user *model.User) error {
+	result := r.db.WithContext(ctx).Create(user)
+	if result.Error != nil {
+		if isDuplicateKeyError(result.Error) {
+			return model.ErrDuplicate
+		}
+		return result.Error
+	}
+	return nil
+}
+
+func (r *userRepository) FindByID(ctx context.Context, id string) (*model.User, error) {
+	var user model.User
+	result := r.db.WithContext(ctx).First(&user, "id = ?", id)
+	if errors.Is(result.Error, gorm.ErrRecordNotFound) {
+		return nil, model.ErrNotFound
+	}
+	if result.Error != nil {
+		return nil, result.Error
+	}
+	return &user, nil
+}
+
+func (r *userRepository) FindByEmail(ctx context.Context, email string) (*model.User, error) {
+	var user model.User
+	result := r.db.WithContext(ctx).First(&user, "email = ?", email)
+	if errors.Is(result.Error, gorm.ErrRecordNotFound) {
+		return nil, model.ErrNotFound
+	}
+	if result.Error != nil {
+		return nil, result.Error
+	}
+	return &user, nil
+}
+
+func (r *userRepository) FindByUsername(ctx context.Context, username string) (*model.User, error) {
+	var user model.User
+	result := r.db.WithContext(ctx).First(&user, "username = ?", username)
+	if errors.Is(result.Error, gorm.ErrRecordNotFound) {
+		return nil, model.ErrNotFound
+	}
+	if result.Error != nil {
+		return nil, result.Error
+	}
+	return &user, nil
+}
+
+func (r *userRepository) Update(ctx context.Context, user *model.User) error {
+	result := r.db.WithContext(ctx).Save(user)
+	if result.Error != nil {
+		if isDuplicateKeyError(result.Error) {
+			return model.ErrDuplicate
+		}
+		return result.Error
+	}
+	return nil
+}
+
+func (r *userRepository) Delete(ctx context.Context, id string) error {
+	result := r.db.WithContext(ctx).Delete(&model.User{}, "id = ?", id)
+	if result.Error != nil {
+		return result.Error
+	}
+	return nil
+}
+
+func (r *userRepository) List(ctx context.Context, offset, limit int) ([]model.User, int64, error) {
+	var users []model.User
+	var total int64
+
+	if err := r.db.WithContext(ctx).Model(&model.User{}).Count(&total).Error; err != nil {
+		return nil, 0, err
+	}
+
+	result := r.db.WithContext(ctx).Offset(offset).Limit(limit).Find(&users)
+	if result.Error != nil {
+		return nil, 0, result.Error
+	}
+
+	return users, total, nil
+}