refactor: move backend files to backend/ directory

Reorganize project structure:
- backend/cmd/openteam/ — entry point
- backend/internal/ — core packages
- backend/middleware/ — HTTP middleware
- backend/router/ — route setup
- backend/wire/ — dependency injection
- backend/pkg/ — shared utilities
- backend/go.mod, go.sum — Go module files

Updated Makefile to work from backend/ directory.
Removed old lowercase makefile.

backend/internal/dao/user.go

@@ -0,0 +1,62 @@
+package dao
+
+import (
+	"opencatd-open/internal/store"
+
+	"gorm.io/gorm"
+)
+
+type UserDAO struct {
+	db *gorm.DB
+}
+
+func NewUserDAO(db *gorm.DB) *UserDAO {
+	return &UserDAO{db: db}
+}
+
+func (d *UserDAO) Create(user *store.User) error {
+	return d.db.Create(user).Error
+}
+
+func (d *UserDAO) GetByID(id uint64) (*store.User, error) {
+	var user store.User
+	err := d.db.First(&user, id).Error
+	if err != nil {
+		return nil, err
+	}
+	return &user, nil
+}
+
+func (d *UserDAO) GetByUsername(username string) (*store.User, error) {
+	var user store.User
+	err := d.db.Where("username = ?", username).First(&user).Error
+	if err != nil {
+		return nil, err
+	}
+	return &user, nil
+}
+
+func (d *UserDAO) GetByEmail(email string) (*store.User, error) {
+	var user store.User
+	err := d.db.Where("email = ?", email).First(&user).Error
+	if err != nil {
+		return nil, err
+	}
+	return &user, nil
+}
+
+func (d *UserDAO) List(limit, offset int) ([]*store.User, int64, error) {
+	var users []*store.User
+	var total int64
+	d.db.Model(&store.User{}).Count(&total)
+	err := d.db.Limit(limit).Offset(offset).Order("created_at DESC").Find(&users).Error
+	return users, total, err
+}
+
+func (d *UserDAO) Update(user *store.User) error {
+	return d.db.Save(user).Error
+}
+
+func (d *UserDAO) Delete(id uint64) error {
+	return d.db.Delete(&store.User{}, id).Error
+}