feat(invoice): enhance invoice update validation and payload construction

backend/api/v1/modules/a24/fa/fa_item_lines/dto.py

@@ -12,7 +12,8 @@ class FaLineItemCreateDTO(BaseModel):
     """DTO para crear una línea de activo fijo"""
 
     # line_item_id references the id in a76.item_lines
-    line_item_id: int = Field(..., description="ID de la línea base en a76.item_lines")
+    # Not required on creation - will be set when the LineItem is created
+    line_item_id: Optional[int] = Field(None, description="ID de la línea base en a76.item_lines")
 
     # Asset information (SCAF specific)
     asset_number: Optional[str] = Field(