Add default admin user and admin-only user creation

Features added:
- Automatic creation of default admin user on first startup (login: admin, password: admin)
- Admin-only endpoint POST /api/admin/users for creating new users
- Admin users can set is_admin flag when creating users
- Non-admin users are blocked from accessing admin endpoints

Implementation:
- Added CreateDefaultAdmin() function in internal/database/database.go
  - Checks if any users exist, creates admin only if database is empty
  - Admin user: login "admin", password "admin", is_admin true
- Added CreateUser() method to auth service for admin user creation
- Added CreateUser() handler to auth handler
- Added /api/admin/users endpoint with AuthMiddleware + AdminMiddleware
- Updated README_GOLANG.md with:
  - Default admin credentials
  - Instructions for creating additional users
  - Admin API documentation

Security:
- Default admin password should be changed after first login
- AdminMiddleware ensures only users with is_admin=true can access admin routes
- Non-admin users receive 403 Forbidden when accessing admin endpoints

Tested:
- Default admin creation on startup ✓
- Admin login with default credentials ✓
- Admin can create new users ✓
- New users can login ✓
- Non-admin users blocked from admin endpoints ✓

internal/services/auth_service.go

@@ -37,6 +37,15 @@ type RegisterRequest struct {
 	LastName  string `json:"last_name"`
 }
 
+// CreateUserRequest represents an admin user creation request
+type CreateUserRequest struct {
+	Login     string `json:"login" binding:"required"`
+	Password  string `json:"password" binding:"required"`
+	FirstName string `json:"first_name"`
+	LastName  string `json:"last_name"`
+	IsAdmin   bool   `json:"is_admin"`
+}
+
 // AuthResponse represents an authentication response
 type AuthResponse struct {
 	Token string       `json:"token"`
@@ -146,3 +155,42 @@ func (s *AuthService) RefreshToken(userID uint) (string, error) {
 
 	return user.Token, nil
 }
+
+// CreateUser creates a new user (admin only)
+func (s *AuthService) CreateUser(req CreateUserRequest) (*models.User, error) {
+	// Check if user already exists
+	var existingUser models.User
+	if err := database.DB.Where("login = ?", req.Login).First(&existingUser).Error; err == nil {
+		return nil, errors.New("user already exists")
+	}
+
+	// Create new user
+	user := models.User{
+		Login:     req.Login,
+		FirstName: req.FirstName,
+		LastName:  req.LastName,
+		IsAdmin:   req.IsAdmin,
+		AuthType:  models.AuthTypeDatabase,
+		Token:     uuid.New().String(),
+	}
+
+	// Set password
+	if err := user.SetPassword(req.Password); err != nil {
+		return nil, err
+	}
+
+	// Save user
+	if err := database.DB.Create(&user).Error; err != nil {
+		return nil, err
+	}
+
+	// Create default preference
+	preference := models.Preference{
+		UserID: user.ID,
+	}
+	if err := database.DB.Create(&preference).Error; err != nil {
+		return nil, err
+	}
+
+	return &user, nil
+}