_parse_dt documents that it returns naive datetimes (CalendarEvent.dtstart is
naive) and every return path strips tz — except the last-resort dateutil
fallback, which returned dateutil's value verbatim. An offset-bearing non-ISO
input (e.g. RFC-2822 'Mon, 05 Jan 2026 14:00:00 +0900', which fromisoformat
rejects but dateutil parses) leaked a tz-aware datetime into the naive dtstart
column via create_event/update_event -> _parse_dt_pair. On read-back,
_expand_rrule compares ev.dtstart against naive window bounds and raised
'can't compare offset-naive and offset-aware datetimes' (500 / no events).
Normalize the fallback to UTC-naive, mirroring the fromisoformat branch. Naive
inputs are unchanged.
ghreprimand
b03b6b91df