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.py

@@ -262,6 +262,15 @@ async def get_obj_li(
             as_list=True,
         )
 
+    if sql_result is False:
+        # Standardized rich error bubbling
+        db_err = format_db_error(get_last_sql_error())
+        
+        # If it's a schema error (like Unknown Column), it's a 400 Bad Request
+        status_code = 400 if db_err.category == "database_schema" else 500
+        
+        return mk_resp(data=False, status_code=status_code, response=response, status_message="Listing failed due to database error.", details=db_err.dict())
+
     if sql_result:
         resp_data_li = [base_name(**record).dict(by_alias=serialization.by_alias, exclude_unset=serialization.exclude_unset, exclude_defaults=serialization.exclude_defaults, exclude_none=serialization.exclude_none) for record in sql_result]
         return mk_resp(data=resp_data_li, response=response)

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)