fix(calendar): keep recurring events with a UTC UNTIL from collapsing to one (#1383)

Events are stored with a naive (UTC) dtstart, but standard .ics exporters
(Google, Apple, Outlook, Fastmail) write the recurrence bound as an absolute
UTC value, e.g. FREQ=DAILY;UNTIL=20240105T090000Z. dateutil refuses to mix a
tz-aware UNTIL with a naive DTSTART ("RRULE UNTIL values must be specified in
UTC when DTSTART is timezone-aware"), so _expand_rrule's except branch swallowed
the ValueError and silently downgraded the event to non-recurring — every
occurrence after the first vanished from the calendar.

When dtstart is naive, strip the trailing Z from UNTIL so it matches the naive
DTSTART before parsing. No effect on tz-aware dtstarts or naive-UNTIL rules.

Adds tests/test_calendar_rrule_until_utc.py — a daily series bounded by a UTC
UNTIL expands to all 5 occurrences (fails before: returns 1, non-recurring).

Co-authored-by: NubsCarson <nubs@nubs.site>

tests/test_calendar_rrule_until_utc.py

@@ -0,0 +1,73 @@
+"""Regression test for RRULE expansion with a UTC UNTIL value.
+
+Standard ICS exporters (Google Calendar, Apple Calendar, Outlook,
+Fastmail) emit recurrence rules of the form
+
+    RRULE:FREQ=DAILY;UNTIL=20240105T090000Z
+
+When such an event is imported, the calendar route stores the event's
+``dtstart`` as a *naive* datetime (the DB column is naive; timed events
+are converted to naive-UTC on import). dateutil >= 2.7 raises
+
+    ValueError: RRULE UNTIL values must be specified in UTC
+                when DTSTART is timezone-aware
+
+whenever the UNTIL is tz-aware (carries a trailing ``Z``) but the
+``dtstart`` is naive. ``_expand_rrule`` catches that ValueError and
+*silently downgrades the event to non-recurring*, so every occurrence
+after the first vanishes from the calendar.
+
+This test pins the correct behaviour: a daily series bounded by a UTC
+UNTIL must expand to all of its occurrences.
+"""
+
+from datetime import datetime
+from types import SimpleNamespace
+
+from tests.test_null_owner_gates import _import_calendar_helpers
+
+
+_MOCK_CAL = SimpleNamespace(name="Personal", color="#5b8abf")
+
+
+def _make_event(**overrides):
+    defaults = {
+        "uid": "evt-until-utc",
+        "summary": "Standup",
+        "dtstart": datetime(2024, 1, 1, 9, 0),
+        "dtend": datetime(2024, 1, 1, 9, 30),
+        "all_day": False,
+        "is_utc": True,
+        "rrule": "",
+        "calendar_id": "cal-001",
+        "color": None,
+        "description": "",
+        "location": "",
+        "event_type": None,
+        "importance": "normal",
+    }
+    defaults.update(overrides)
+    ev = SimpleNamespace(**defaults)
+    ev.calendar = _MOCK_CAL
+    return ev
+
+
+def test_expand_rrule_with_utc_until_keeps_all_occurrences():
+    """FREQ=DAILY;UNTIL=...Z must expand to every occurrence, not collapse
+    to a single non-recurring event."""
+    cal = _import_calendar_helpers()
+    ev = _make_event(rrule="FREQ=DAILY;UNTIL=20240105T090000Z")
+
+    results = cal._expand_rrule(ev, datetime(2024, 1, 1), datetime(2024, 1, 10))
+
+    # Jan 1, 2, 3, 4, 5 — five daily occurrences up to and including UNTIL.
+    assert len(results) == 5, (
+        f"Expected 5 daily occurrences bounded by UTC UNTIL, got "
+        f"{len(results)}: {[r['uid'] for r in results]}"
+    )
+    assert all(r["is_recurrence"] is True for r in results), (
+        "Occurrences must be flagged as recurrences, not silently downgraded "
+        f"to non-recurring: {[(r['uid'], r['is_recurrence']) for r in results]}"
+    )
+    assert results[0]["uid"] == "evt-until-utc::2024-01-01T09:00"
+    assert results[-1]["uid"] == "evt-until-utc::2024-01-05T09:00"