fix(caldav): point-event round-trip, recurrence push, delete propagation

Drift-audit Group 5 #7/#8 + Group 7 (CalDAV write-path):

- Point events no longer fabricate a 60-min DTEND: caldav.create_event emits
  DTSTART-only when there's no end and no duration, so the next pull doesn't
  read it back as duration_minutes=60 and silently lengthen the event.
- Recurrence edits now propagate: caldav.update_event gains a recurrence param
  (sentinel = leave unchanged; value/empty = set/clear RRULE), and _push_update
  passes the local event's rule so a changed/cleared RRULE isn't overwritten
  by the stale remote rule on the next pull.
- Event deletions propagate to CalDAV: trash.delete captures an event's
  caldav_uid before soft-deleting and fires _push_delete, so a UI/MCP delete
  removes the remote copy instead of leaving it to linger. (delete_event the
  service primitive is kept — still tested/usable — rather than removed.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/fabledassistant/services/events.py

@@ -450,6 +450,9 @@ async def _push_update(event: Event, user_id: int, old_title: str = "") -> None:
             end=derived_end.isoformat() if derived_end else None,
             description=event.description or None,
             location=event.location or None,
+            # Propagate the (possibly cleared) RRULE so a local recurrence edit
+            # isn't overwritten by the stale remote rule on the next pull.
+            recurrence=event.recurrence,
         )
     except Exception:
         logger.warning("CalDAV push (update) failed for event %d", event.id, exc_info=True)