feat: 유저 관리 API 추가 (목록 조회, 권한 변경, 삭제)

- GET /api/users - 전체 유저 목록 (admin only)
- PATCH /api/users/:id/role - 권한 변경 (admin only)
- DELETE /api/users/:id - 유저 삭제 (admin only)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/auth/repository.go

@@ -19,3 +19,23 @@ func (r *Repository) FindByUsername(username string) (*User, error) {
 func (r *Repository) Create(user *User) error {
 	return r.db.Create(user).Error
 }
+
+func (r *Repository) FindAll() ([]User, error) {
+	var users []User
+	err := r.db.Order("created_at asc").Find(&users).Error
+	return users, err
+}
+
+func (r *Repository) FindByID(id string) (*User, error) {
+	var user User
+	err := r.db.First(&user, id).Error
+	return &user, err
+}
+
+func (r *Repository) UpdateRole(id string, role Role) error {
+	return r.db.Model(&User{}).Where("id = ?", id).Update("role", role).Error
+}
+
+func (r *Repository) Delete(id string) error {
+	return r.db.Delete(&User{}, id).Error
+}