Admin routes: quest definitions

The admin plane gains quest-definition routes on the existing admin
router, mirroring the children routes' thin-adapter pattern: the body
model carries the shape, the handler builds ONE ScheduleRule from the
request's rule object through the recurrence model's own from_dict
(rule validation stays the model's construction-time job), and every
call goes to ONE nestquest_core.quest_definitions business function —
the title/window/assignee policies, the whole-set window replacement
and the never-hard-delete policy all live in the core, never in a
handler.

- POST /quest-definitions: title + rule + assignees + windows (+
  optional description/icon), persisted atomically by the core.
- PATCH /quest-definitions/{id}: only supplied fields are forwarded
  (exclude_unset); an explicit null description/icon clears it, a null
  rule/windows is rejected; assignment stays unsettable here.
- PATCH /quest-definitions/{id}/active: the only removal path — the
  flag flips, the row, rule, assignees and windows survive.

Error mapping stays deliberately narrow: the core's definition_id:-
prefixed not-exist error is 404; every other ValueError (including
RuleValidationError) is 422 with the core's message, so a rejected
rule, window, assignee or title from the BODY never disguises itself
as a missing resource and an unexpected error is never swallowed as a
4xx.

Task: b586a2f1-1532-4194-a19a-8f8386d22590 (Feature 16).

api/nestquest_core.py

@@ -128,6 +128,20 @@ core_settings = importlib.import_module(f"{_CORE_MODULE_NAME}.settings")
 #: the admin plane's children routes call.
 core_children = importlib.import_module(f"{_CORE_MODULE_NAME}.children")
 
+#: The quest-definition business layer (``core.quest_definitions`` —
+#: ``create_quest_definition``, ``edit_quest_definition``,
+#: ``set_quest_definition_active``) the admin plane's quest-definition
+#: routes call.
+core_quest_definitions = importlib.import_module(
+    f"{_CORE_MODULE_NAME}.quest_definitions"
+)
+
+#: The recurrence model (``core.recurrence.ScheduleRule`` and
+#: ``RuleValidationError``) the quest-definition routes build request
+#: rules with — the model validates at construction, the routes never
+#: re-implement rule validation.
+core_recurrence = importlib.import_module(f"{_CORE_MODULE_NAME}.recurrence")
+
 __all__ = [
     "core",
     "core_children",
@@ -135,6 +149,8 @@ __all__ = [
     "core_db",
     "core_events",
     "core_migrations",
+    "core_quest_definitions",
+    "core_recurrence",
     "core_snapshot",
     "core_settings",
 ]