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_household_clock.py

@@ -32,7 +32,7 @@ from zoneinfo import ZoneInfo
 
 import pytest
 
-from api import routes_admin, routes_panel
+from api import host_zone, routes_admin, routes_panel
 from api import scheduler as scheduler_module
 from api.database import _executor
 from api.scheduler import MissedSweepScheduler, _delay_seconds
@@ -632,22 +632,45 @@ async def test_admin_saved_zone_is_authoritative_across_surfaces(
     assert delays[1] == pytest.approx(299)
 
 
-async def test_panel_snapshot_publishes_the_stored_zone(
-    household: SimpleNamespace,
+async def test_empty_zone_publishes_the_api_host_zone_to_panel_and_admin(
+    household: SimpleNamespace, monkeypatch
 ) -> None:
-    """The panel snapshot carries the stored application zone verbatim so
-    the cards format their visible date and clock in it — ``""`` until the
-    admin saves one, then exactly the saved zone."""
-    async def panel_zone() -> str:
-        response = await household.client.get(
-            "/api/v1/panel/snapshot", headers=_panel_headers()
-        )
-        assert response.status_code == 200
-        return response.json()["timezone"]
+    """With no zone stored, the API reads the host's local time — and the
+    panel snapshot and the admin settings both publish that host's own
+    zone, so the cards and the admin app format in the API's zone, never
+    Home Assistant's or the browser's.
 
-    assert await panel_zone() == ""
-    await _set_zone(household, "Pacific/Auckland")
-    assert await panel_zone() == "Pacific/Auckland"
+    The host runs Pacific/Auckland: 12:30 UTC on the 26th is already
+    00:30 on the 27th there, so the API's household day is the 27th while
+    a UTC Home Assistant or browser would still read the 26th.  Once the
+    admin saves a zone, both publish exactly that zone.
+    """
+    host = "Pacific/Auckland"
+    monkeypatch.setattr(host_zone, "host_timezone", lambda: host)
+    household.clock["now"] = _utc(12, 30).astimezone(ZoneInfo(host))
+
+    panel = await household.client.get(
+        "/api/v1/panel/snapshot", headers=_panel_headers()
+    )
+    assert panel.status_code == 200
+    settings = await _admin_settings(household)
+    assert settings["timezone"] == ""
+    assert panel.json()["timezone"] == settings["effective_timezone"] == host
+    # The published zone reproduces the API's own household day.
+    assert panel.json()["today_iso"] == "2026-09-27"
+    assert (
+        _utc(12, 30).astimezone(ZoneInfo(host)).date().isoformat()
+        == panel.json()["today_iso"]
+    )
+
+    await _set_zone(household, "America/Chicago")
+    panel = await household.client.get(
+        "/api/v1/panel/snapshot", headers=_panel_headers()
+    )
+    settings = await _admin_settings(household)
+    assert settings["timezone"] == "America/Chicago"
+    assert panel.json()["timezone"] == settings["effective_timezone"]
+    assert settings["effective_timezone"] == "America/Chicago"
 
 
 # --- settings writes are serialised with planning and sweeping (TZ-003) ------