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/database/database.go

@@ -91,3 +91,55 @@ func Close() error {
 func GetDB() *gorm.DB {
 	return DB
 }
+
+// CreateDefaultAdmin creates a default admin user if no users exist
+func CreateDefaultAdmin() error {
+	if DB == nil {
+		return fmt.Errorf("database not initialized")
+	}
+
+	// Check if any users exist
+	var count int64
+	if err := DB.Model(&models.User{}).Count(&count).Error; err != nil {
+		return fmt.Errorf("failed to count users: %w", err)
+	}
+
+	// If users exist, don't create default admin
+	if count > 0 {
+		log.Println("Users already exist, skipping default admin creation")
+		return nil
+	}
+
+	log.Println("Creating default admin user (login: admin, password: admin)")
+
+	// Create default admin user
+	admin := models.User{
+		Login:     "admin",
+		FirstName: "Admin",
+		LastName:  "User",
+		IsAdmin:   true,
+		AuthType:  models.AuthTypeDatabase,
+		Token:     "default-admin-token",
+	}
+
+	// Set password
+	if err := admin.SetPassword("admin"); err != nil {
+		return fmt.Errorf("failed to set admin password: %w", err)
+	}
+
+	// Save admin user
+	if err := DB.Create(&admin).Error; err != nil {
+		return fmt.Errorf("failed to create admin user: %w", err)
+	}
+
+	// Create default preference for admin
+	preference := models.Preference{
+		UserID: admin.ID,
+	}
+	if err := DB.Create(&preference).Error; err != nil {
+		return fmt.Errorf("failed to create admin preference: %w", err)
+	}
+
+	log.Printf("Default admin user created successfully (ID: %d)", admin.ID)
+	return nil
+}