Publish the API's effective timezone and tolerate an older snapshot payload

TZ-002: with no zone stored the API reads its clock in the host's local
time, but the panel fell back to Home Assistant's zone and the admin app
to the browser's, so dates could disagree near midnight. The API now
names the zone it actually uses (api/host_zone.py: the stored zone, else
the host's own IANA zone from TZ, /etc/localtime or /etc/timezone) and
publishes it as the panel snapshot's `timezone` and the admin settings'
read-only `effective_timezone`. Both cards and the admin app format in
that one zone; Home Assistant's or the browser's zone applies only when
none is published.

TZ-003: snapshot_from_api_payload reads `timezone` with a "" default, so
an integration upgraded ahead of its API keeps refreshing against the
pre-change payload.

Task 7e173881-fb99-426f-a2f0-2678439ffc39

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

tests/test_api_admin_snapshot.py

@@ -37,7 +37,7 @@ from zoneinfo import ZoneInfo
 
 import pytest
 
-from api import routes_admin, routes_panel
+from api import host_zone, routes_admin, routes_panel
 from api.nestquest_core import core_snapshot
 from tests.admin_jwt_harness import (
     ADMIN_KEY,
@@ -101,6 +101,8 @@ async def snapshot_household(
 
     monkeypatch.setattr(routes_admin, "_local_now", _admin_now)
     monkeypatch.setattr(routes_panel, "_local_now", lambda: pinned_now)
+    # The API host runs UTC, the zone of the pinned clock.
+    monkeypatch.setattr(host_zone, "host_timezone", lambda: "UTC")
     runner = AdminRunner(temp_db_path, jwks_for(ADMIN_KEY, KID))
     async with runner as client:
         yield SimpleNamespace(
@@ -166,8 +168,9 @@ async def test_admin_jwt_returns_documented_snapshot(
     Every field is asserted as an explicit literal, with ONE clock
     read for the request.  With no missed view in the real snapshot,
     the admin body equals the panel body for the same household — the
-    panel body additionally carries the stored ``timezone`` its cards
-    format the visible date and clock in (unset here).
+    panel body additionally carries the effective ``timezone`` its cards
+    format the visible date and clock in (none stored here: the UTC
+    host's own zone).
     """
     client = snapshot_household.client
     seed = snapshot_household.seed
@@ -243,7 +246,7 @@ async def test_admin_jwt_returns_documented_snapshot(
         "/api/v1/panel/snapshot", headers=_panel_headers()
     )
     assert panel.status_code == 200
-    assert panel.json() == {**body, "timezone": ""}
+    assert panel.json() == {**body, "timezone": "UTC"}
 
 
 async def test_admin_keeps_missed_instance_panel_omits_it(