Clean up label-embedding architecture

- Move {id,label} stripping into _SetCriterion.from_json() so both
  MultiCriterion and ChoiceCriterion normalise at the parse boundary;
  the querying layer stays typed (list[int] / list[str]) and clean.
- Revert MultiCriterion to a thin _extra_q() override; _SetCriterion.to_q()
  is no longer duplicated.
- JS: readSearchSelect always emits {id, label} objects — no conditional
  mixed-type arrays. filter_bar.js stores them as-is for all fields,
  removing the fragile isIdField hardcoded list.
- Update tests to use the {id, label} filter format.

https://claude.ai/code/session_01EyAJcMoDktLrY9tSbdHViA

games/static/js/search_select.js

@@ -436,11 +436,10 @@
             }
             var value = pill.getAttribute("data-value");
             var label = pill.getAttribute("data-label") || "";
-            var entry = label ? {id: value, label: label} : value;
             if (pill.getAttribute("data-search-select-type") === "exclude") {
-              excluded.push(entry);
+              excluded.push({id: value, label: label});
             } else {
-              included.push(entry);
+              included.push({id: value, label: label});
             }
           });
         }