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_host_zone.py

@@ -0,0 +1,71 @@
+"""The API host's zone and the effective application timezone (task 7e173881).
+
+With no zone stored the API reads its clock in the host's local time, so
+:func:`api.host_zone.effective_timezone` names that host zone — resolved
+the way the C library resolves local time — for the panel and admin app
+to format in.  Every host configuration here is a temporary file tree.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+from api import host_zone
+
+
+def _host(tmp_path: Path, *, link: str | None = None, name: str | None = None):
+    """A fake ``/etc``: ``localtime`` linked into a zoneinfo tree at
+    ``link``, and/or an ``/etc/timezone`` file holding ``name``."""
+    localtime = tmp_path / "etc" / "localtime"
+    timezone_file = tmp_path / "etc" / "timezone"
+    localtime.parent.mkdir(parents=True)
+    if link is not None:
+        target = tmp_path / "usr" / "share" / "zoneinfo" / link
+        target.parent.mkdir(parents=True)
+        target.write_bytes(b"TZif")
+        localtime.symlink_to(target)
+    if name is not None:
+        timezone_file.write_text(f"{name}\n", encoding="utf-8")
+    return {"timezone_file": timezone_file, "localtime": localtime}
+
+
+def test_tz_environment_variable_wins(tmp_path: Path) -> None:
+    paths = _host(tmp_path, link="Europe/Paris")
+    assert host_zone.host_timezone({"TZ": "Asia/Kolkata"}, **paths) == "Asia/Kolkata"
+    assert host_zone.host_timezone({"TZ": ":Asia/Kolkata"}, **paths) == "Asia/Kolkata"
+    assert host_zone.host_timezone({"TZ": ""}, **paths) == "UTC"
+
+
+def test_localtime_link_names_the_zone(tmp_path: Path) -> None:
+    paths = _host(tmp_path, link="America/Argentina/Buenos_Aires")
+    assert host_zone.host_timezone({}, **paths) == "America/Argentina/Buenos_Aires"
+
+
+def test_etc_timezone_names_a_copied_localtime(tmp_path: Path) -> None:
+    paths = _host(tmp_path, name="Pacific/Auckland")
+    paths["localtime"].write_bytes(b"TZif")
+    assert host_zone.host_timezone({}, **paths) == "Pacific/Auckland"
+
+
+def test_unconfigured_host_runs_utc(tmp_path: Path) -> None:
+    assert host_zone.host_timezone({}, **_host(tmp_path)) == "UTC"
+
+
+@pytest.mark.parametrize("tz", ["EST+5", "Not/AZone", ":/etc/localtime"])
+def test_an_unnamed_host_zone_is_empty(tmp_path: Path, tz: str) -> None:
+    assert host_zone.host_timezone({"TZ": tz}, **_host(tmp_path)) == ""
+
+
+def test_unnamed_copied_localtime_is_empty(tmp_path: Path) -> None:
+    paths = _host(tmp_path)
+    paths["localtime"].write_bytes(b"TZif")
+    assert host_zone.host_timezone({}, **paths) == ""
+
+
+def test_effective_timezone_is_the_stored_zone_else_the_host_zone(
+    monkeypatch,
+) -> None:
+    monkeypatch.setattr(host_zone, "host_timezone", lambda: "Europe/Dublin")
+    assert host_zone.effective_timezone("America/Chicago") == "America/Chicago"
+    assert host_zone.effective_timezone("") == "Europe/Dublin"