get_calendar_events() accepted start/end datetime parameters but called
calendar.events() which fetches all events, silently discarding the
date filters. This caused nc_calendar_list_events and
nc_calendar_get_upcoming_events to return the entire calendar history.
Add _search_events_by_date() helper that builds a CalDAV REPORT query
with a <time-range> filter (RFC 4791 §9.9) for server-side filtering.
Falls back to calendar.events() when no dates are given.
Closes#538
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
