Accept assignee_child_ids on the admin definition edit route

PATCH /api/v1/admin/quest-definitions/{id} now takes an optional
assignee_child_ids list that replaces the definition's assignee set.
The replace runs inside the core edit's single DAO transaction, so a
rejected list (empty, duplicate, unknown or inactive child) returns 422
and writes nothing, including any other field in the same body. The
existing single regeneration path rebuilds future instances. An omitted
field leaves assignees unchanged.

Task: 1c8096f6-643d-441b-a61a-892e0d5b3b59

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_api_admin_questdef.py

@@ -18,8 +18,9 @@ caveat in api/nestquest_core.py):
   and windows exactly as posted.
 - ``PATCH /api/v1/admin/quest-definitions/{id}``: an edit PERSISTS the
   supplied fields and leaves omitted fields untouched (exclude_unset
-  semantics); a supplied ``windows`` list replaces the whole set;
-  assignment is not settable through the edit.
+  semantics); a supplied ``windows`` list replaces the whole set; an
+  omitted ``assignee_child_ids`` leaves the assignees untouched (the
+  supplied-list cases live in test_api_admin_questdef_assignees.py).
 - ``PATCH /api/v1/admin/quest-definitions/{id}/active`` with
   ``is_active: false`` sets the flag FALSE and the definition STILL
   EXISTS with its rule, assignees and windows (never hard-deleted);
@@ -397,7 +398,7 @@ async def test_edit_persists_supplied_fields_and_leaves_omitted_untouched(
     )
     assert stored.definition.icon == "toy-box"
     assert stored.rule.to_dict() == _weekly_rule_dict()
-    # Assignment is not settable through the edit route: untouched.
+    # assignee_child_ids was omitted: the assignees are untouched.
     assert sorted(child.id for child in stored.assignees) == [ada, bo]
 
     # A second edit touching only the rule leaves the metadata and the