refactor(models): make first_name optional in SystemUserModel

To align with the optional nature of last_name and phone fields, first_name is now also optional. This provides more flexibility when creating user records where first name might not be required.

app/system_users/models/model.py

@@ -61,7 +61,7 @@ class SystemUserModel(BaseModel):
     password_hash: str = Field(..., description="Bcrypt hashed password")
     
     # Personal information
-    first_name: str = Field(..., description="User first name")
+    first_name: Optional[str] = Field(None, description="User first name")
     last_name: Optional[str] = Field(None, description="User last name")
     phone: Optional[str] = Field(None, description="User phone number (E.164 format)")
     
@@ -127,4 +127,4 @@ class SystemUserModel(BaseModel):
                 "created_by": "system",
                 "created_at": "2024-01-15T08:00:00Z"
             }
-        }
+        }