fix(customers): Auto-generate username when creating customers

The CustomerSerializer was missing a create method to generate a unique
username, causing IntegrityError when trying to create customers.

- Add first_name and last_name as write-only fields
- Remove email from read_only_fields so it can be set on creation
- Generate username from email prefix (with counter for uniqueness)
- Fall back to UUID-based username if no email provided

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

smoothschedule/schedule/serializers.py

@@ -38,6 +38,8 @@ class ResourceTypeSerializer(serializers.ModelSerializer):
 class CustomerSerializer(serializers.ModelSerializer):
     """Serializer for Customer (User with role=CUSTOMER)"""
     name = serializers.SerializerMethodField()
+    first_name = serializers.CharField(write_only=True, required=False, allow_blank=True)
+    last_name = serializers.CharField(write_only=True, required=False, allow_blank=True)
     total_spend = serializers.SerializerMethodField()
     last_visit = serializers.SerializerMethodField()
     status = serializers.SerializerMethodField()
@@ -52,11 +54,29 @@ class CustomerSerializer(serializers.ModelSerializer):
     class Meta:
         model = User
         fields = [
-            'id', 'name', 'email', 'phone', 'city', 'state', 'zip',
+            'id', 'name', 'first_name', 'last_name', 'email', 'phone', 'city', 'state', 'zip',
             'total_spend', 'last_visit', 'status', 'avatar_url', 'tags',
             'user_id', 'user_data',
         ]
-        read_only_fields = ['id', 'email']
+        read_only_fields = ['id']
+
+    def create(self, validated_data):
+        """Create a customer with auto-generated username"""
+        import uuid
+        email = validated_data.get('email', '')
+        # Generate username from email or use a UUID if no email
+        if email:
+            base_username = email.split('@')[0]
+            username = base_username
+            counter = 1
+            while User.objects.filter(username=username).exists():
+                username = f"{base_username}{counter}"
+                counter += 1
+        else:
+            username = f"customer_{uuid.uuid4().hex[:8]}"
+
+        validated_data['username'] = username
+        return super().create(validated_data)
 
     def get_name(self, obj):
         return obj.full_name