fix(02): correct DURATION-only event end in expand.ts (BUG 1)

- Replace dtend ?? dtstart with event.endDate which handles DURATION-only VEVENTs
- Add positive-duration guard (PT30M / P1D) to both non-recurring and recurring branches
- Add single-duration.ics fixture and regression test asserting end > start for DURATION-only events

apps/api/tests/fixtures/single-duration.ics

@@ -0,0 +1,28 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//FamilySync//Test//EN
+BEGIN:VTIMEZONE
+TZID:America/Toronto
+BEGIN:STANDARD
+DTSTART:19701101T020000
+RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=11
+TZOFFSETFROM:-0400
+TZOFFSETTO:-0500
+TZNAME:EST
+END:STANDARD
+BEGIN:DAYLIGHT
+DTSTART:19700308T020000
+RRULE:FREQ=YEARLY;BYDAY=2SU;BYMONTH=3
+TZOFFSETFROM:-0500
+TZOFFSETTO:-0400
+TZNAME:EDT
+END:DAYLIGHT
+END:VTIMEZONE
+BEGIN:VEVENT
+UID:single-duration-only@familysync.test
+DTSTART;TZID=America/Toronto:20260618T080000
+DURATION:PT1H
+SUMMARY:Morning Standup
+DESCRIPTION:Non-recurring timed event with DURATION only, no DTEND
+END:VEVENT
+END:VCALENDAR