Buchungskalender: iCal-Import und -Export

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

test/fixtures/ical/airbnb.ics

@@ -0,0 +1,18 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//Airbnb Inc//Hosting Calendar 1.0//EN
+BEGIN:VEVENT
+DTSTART;VALUE=DATE:20261120
+DTEND;VALUE=DATE:20261125
+DTSTAMP:20261001T120000Z
+UID:a1-xyz@airbnb.com
+SUMMARY:Reserved
+DESCRIPTION:Reservation URL: https://www.airbnb.com/hosting/reservations/details/HM123\nPhone Number (Last 4 Digits): 1234
+END:VEVENT
+BEGIN:VEVENT
+DTSTART;VALUE=DATE:20261201
+DTEND;VALUE=DATE:20261202
+UID:a2@airbnb.com
+SUMMARY:Airbnb (Not available)
+END:VEVENT
+END:VCALENDAR