Make last_name optional and add default for first_name.

Updated the schemas and models to make `last_name` optional and set a default value of "user" for `first_name`. This change enhances flexibility for user data handling and ensures proper defaults when values are not provided.

backend/app/schemas/users.py

@@ -10,7 +10,7 @@ from pydantic import BaseModel, EmailStr, field_validator, ConfigDict
 class UserBase(BaseModel):
     email: EmailStr
     first_name: str
-    last_name: str
+    last_name: Optional[str] = None
     phone_number: Optional[str] = None
 
     @field_validator('phone_number')