Serialise time zone adoption with the missed-sweep scheduler

The wake flag compared an adoption only against the plan the loop last
recorded, so New York -> UTC -> New York during the re-plan read left
the scheduler sleeping on UTC midnight; and an adoption committing
between the pre-sweep check and the sweep still let the stale sweep run.

`MissedSweepScheduler.settings_lock` now serialises the panel's adoption
(write plus `settings_changed`) with the loop's plan read and with its
pre-sweep re-read plus sweep.  It is never held across the wait, and the
loop holds nothing else while acquiring it, so a panel request is at
most delayed by one settings read or one sweep.

`_sleep_unless_woken` now cancels and awaits both child tasks, and
re-raises a failed sleep inside the loop's retry block, so the backoff
runs instead of a sweep and nothing outlives `stop()`.

Tests (all four fail on bcf387d): the successive-zone race, an adoption
issued during the pre-sweep check, a failing sleep, and stop() waiting
for the sleep's cancellation cleanup.

Task: 6351b36d-0eef-4e7d-bf5a-96b754cbc883

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

api/routes_panel.py

@@ -200,17 +200,21 @@ async def _household_clock(
     (:func:`nestquest_core.settings_store.adopt_reported_timezone`), so
     the admin plane follows it too, and the already-sleeping missed-sweep
     scheduler is told (:meth:`MissedSweepScheduler.settings_changed`) so
-    it re-plans for the household rollover.  An invalid
+    it re-plans for the household rollover — under the scheduler's
+    ``settings_lock``, so the adoption never lands between its settings
+    check and its sweep.  An invalid
     reported zone is logged and ignored — the panel keeps serving on
     the stored settings.
     """
     reported = request.headers.get(PANEL_TIMEZONE_HEADER, "").strip()
     if reported:
+        scheduler = request.app.state.sweep_scheduler
         try:
-            settings = await core_settings_store.adopt_reported_timezone(
-                database, reported
-            )
-            request.app.state.sweep_scheduler.settings_changed(settings)
+            async with scheduler.settings_lock:
+                settings = await core_settings_store.adopt_reported_timezone(
+                    database, reported
+                )
+                scheduler.settings_changed(settings)
         except ValueError:
             LOGGER.warning(
                 "Ignoring invalid %s header value %r",