feat(api): standardize V3 error status codes and documentation

- Updated V3 CRUD routers to return 400 Bad Request for database schema
  errors (unknown columns) across all list and search endpoints.
- Fixed serialization issue in nested patch endpoint.
- Overhauled Section 7 of Frontend Integration Guide to document HTTP
  status code mappings for common error categories.

app/routers/api_crud_v3_nested.py

@@ -310,7 +310,7 @@ async def patch_child_obj(
         return mk_resp(data=True, response=response, status_message="Updated successfully.")
     else:
         db_err = format_db_error(get_last_sql_error())
-        return mk_resp(data=False, status_code=400, response=response, status_message="Update failed.", details=db_err)
+        return mk_resp(data=False, status_code=400, response=response, status_message="Update failed.", details=db_err.dict())
 
 
 @router.delete('/{parent_obj_type}/{parent_obj_id}/{child_obj_type}/{child_obj_id}', response_model=Resp_Body_Base)