Add core presence write layer and admin presence routes

The presence business operations (schedule upsert, override
create/delete, each followed by regenerate_for_child) lived only in the
HA-coupled service handlers, so the API admin plane could not reuse
them.  Extract them into core/presence_management.py — model-validated
PresenceSchedule/PresenceOverride construction, the typed DAO calls
and the regeneration side effect in ONE Home-Assistant-free
implementation — and reduce the three HA service handlers to thin
wrappers over it.

Expose the admin plane's presence routes on the existing admin router
(PUT /children/{child_id}/presence-schedule, POST
/presence-overrides, DELETE /presence-overrides/{override_id}): the
body models carry only the shape, validation stays in the presence
model, and the error mapping stays narrow (unknown child/override 404,
rejected schedule/override 422).  Tests drive all three routes as an
admin, prove the schedule upserts in place, an override is created and
then deleted, and the child's resolved presence (PresenceEngine)
changes accordingly.

api/nestquest_core.py

@@ -142,13 +142,35 @@ core_quest_definitions = importlib.import_module(
 #: re-implement rule validation.
 core_recurrence = importlib.import_module(f"{_CORE_MODULE_NAME}.recurrence")
 
+#: The presence model (``core.presence.PresenceEngine``,
+#: ``PresenceSchedule``, ``PresenceOverride``) the admin plane's
+#: presence routes serialize and the tests resolve presence with.
+core_presence = importlib.import_module(f"{_CORE_MODULE_NAME}.presence")
+
+#: The presence-write business layer
+#: (``core.presence_management.set_presence_schedule`` et al.) the
+#: admin plane's presence routes call — the ONE implementation the HA
+#: services share.
+core_presence_management = importlib.import_module(
+    f"{_CORE_MODULE_NAME}.presence_management"
+)
+
+#: The typed presence DAOs (``core.dao_presence``) the admin-plane tests
+#: read stored schedules and overrides back through.
+core_dao_presence = importlib.import_module(
+    f"{_CORE_MODULE_NAME}.dao_presence"
+)
+
 __all__ = [
     "core",
     "core_children",
     "core_completion",
+    "core_dao_presence",
     "core_db",
     "core_events",
     "core_migrations",
+    "core_presence",
+    "core_presence_management",
     "core_quest_definitions",
     "core_recurrence",
     "core_snapshot",