Strip tz in _parse_dt dateutil fallback (naive-datetime contract) (#2557)

_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.

(cherry picked from commit b03b6b91df)

Co-authored-by: ghreprimand <203024559+ghreprimand@users.noreply.github.com>

routes/calendar_routes.py

@@ -399,7 +399,17 @@ def _parse_dt(s: str) -> datetime:
     # Last resort: dateutil's fuzzy parser
     try:
         from dateutil import parser as _du
-        return _du.parse(s)
+        parsed = _du.parse(s)
+        # Strip tz like every other return path above — this function's
+        # contract is naive datetimes (CalendarEvent.dtstart is naive). An
+        # offset-bearing non-ISO input (e.g. RFC-2822 "Mon, 05 Jan 2026
+        # 14:00:00 +0900") otherwise leaked tz-aware into the naive column and
+        # crashed read-back comparisons in _expand_rrule with "can't compare
+        # offset-naive and offset-aware datetimes".
+        if parsed.tzinfo is not None:
+            from datetime import timezone as _tz
+            return parsed.astimezone(_tz.utc).replace(tzinfo=None)
+        return parsed
     except Exception:
         raise ValueError(f"could not parse datetime: {s!r}")