Buchungskalender: iCal-Import und -Export

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/fixtures/ical/booking.ics

@@ -0,0 +1,17 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//Booking.com//EN
+CALSCALE:GREGORIAN
+BEGIN:VEVENT
+DTSTART;VALUE=DATE:20261110
+DTEND;VALUE=DATE:20261113
+UID:b1@booking.com
+SUMMARY:CLOSED - Not available
+END:VEVENT
+BEGIN:VEVENT
+DTSTART;VALUE=DATE:20261201
+DTEND;VALUE=DATE:20261203
+UID:b2@booking.com
+SUMMARY:CLOSED - Not available
+END:VEVENT
+END:VCALENDAR