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>

api/routes_admin.py

@@ -30,10 +30,10 @@ hand-rolled here), and the call goes to
 ``set_quest_definition_active``.  The title policy, the window-name
 and due-time policy, the assignee checks, the whole-set window
 replacement and the never-hard-delete policy (deactivation is the only
-removal path) all live in the core.  Assignment is NOT settable
-through the API: the core's edit path takes no assignees.  The edit
-route forwards only SUPPLIED fields (``exclude_unset``), like the
-children edit.
+removal path) all live in the core.  The edit route forwards only
+SUPPLIED fields (``exclude_unset``), like the children edit; a supplied
+``assignee_child_ids`` list REPLACES the whole assignee set inside the
+core edit's one transaction (and its one regeneration).
 
 The presence routes follow the same pattern over
 :mod:`nestquest_core.presence_management` — the ONE presence write
@@ -182,8 +182,9 @@ deliberately narrow):
   set_active on an unknown id) is mapped to 404.
 - A core ``ValueError`` reporting a NON-EXISTENT DEFINITION (edit or
   set_active on an unknown id — the core prefixes that error with
-  ``definition_id:``) is mapped to 404.  Create's rejected-assignee
-  error names a child from the BODY, not a path id, so it maps to 422.
+  ``definition_id:``) is mapped to 404.  Create's and edit's
+  rejected-assignee error names a child from the BODY, not a path id,
+  so it maps to 422.
 - Every OTHER core ``ValueError`` — an empty display name, an explicit
   ``null`` colour/avatar_ref (clearing is not supported), a reorder
   list that is not a complete permutation of the children (missing,
@@ -476,11 +477,12 @@ class AdminQuestDefinitionEditRequest(BaseModel):
     field stays unchanged.  An explicit ``null`` ``description``/
     ``icon`` IS passed and CLEARS the stored value (the core supports
     clearing here, unlike the children edit); an explicit ``null``
-    ``rule`` or ``windows`` is passed and rejected by the core.  A
-    supplied ``windows`` list REPLACES the whole window set, and
-    ``assignees`` are deliberately absent — assignment is not settable
-    through this route.  An omitted (or ``null``) ``skip_on_away``
-    keeps the stored value.
+    ``rule``, ``windows`` or ``assignee_child_ids`` is passed and
+    rejected by the core.  A supplied ``windows`` list REPLACES the
+    whole window set, and a supplied ``assignee_child_ids`` list
+    REPLACES the whole assignee set (non-empty, every id an existing
+    active child — create's contract; a rejection changes nothing).  An
+    omitted (or ``null``) ``skip_on_away`` keeps the stored value.
     """
 
     title: str | None = None
@@ -489,6 +491,7 @@ class AdminQuestDefinitionEditRequest(BaseModel):
     rule: AdminRuleRequest | None = None
     windows: list[str | tuple[str, str | None]] | None = None
     skip_on_away: StrictBool | None = None
+    assignee_child_ids: list[int] | None = None
 
 
 class AdminQuestDefinitionActiveRequest(BaseModel):
@@ -1266,10 +1269,12 @@ async def admin_edit_quest_definition(
     field is never passed to
     :func:`nestquest_core.quest_definitions.edit_quest_definition` and
     stays unchanged; a supplied ``windows`` list replaces the WHOLE
-    window set in the core, and a supplied ``rule`` is rebuilt into a
-    ScheduleRule (:func:`_rule_from_request`, validated by the model).
-    An explicit ``null`` ``rule``/``windows`` reaches the core as None
-    and is rejected there (422), while an explicit ``null``
+    window set in the core, a supplied ``assignee_child_ids`` list
+    replaces the WHOLE assignee set in the same core transaction, and a
+    supplied ``rule`` is rebuilt into a ScheduleRule
+    (:func:`_rule_from_request`, validated by the model).  An explicit
+    ``null`` ``rule``/``windows``/``assignee_child_ids`` reaches the
+    core as None and is rejected there (422), while an explicit ``null``
     ``description``/``icon`` clears the stored value — the core edit
     path supports clearing, unlike the children edit.  An unknown
     definition id is mapped to 404 and any other core ``ValueError``