feat(calendar): Live-Refresh ueber CalDAV, Tagklick-Navigation, Listen-Ansicht

- caldav.py sendet SSE-Notifications bei Event-PUT/DELETE und Kalender-Loeschung,
  damit das Web-UI auch auf Aenderungen aus DAVx5 sofort reagiert.
- FullCalendar navLinks: Klick auf Tagesnummer im Monatsraster wechselt in
  die Tagesansicht.
- Neue Listen-Ansicht mit Volltext-Suche, Datumsbereich, Kalender-Filter,
  Sortierung nach Datum/Titel und Loeschen-Button pro Zeile.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/dav/caldav.py

@@ -26,8 +26,14 @@ from functools import wraps
 from flask import Response, request
 
 from app.extensions import db
-from app.models.calendar import Calendar, CalendarEvent
+from app.models.calendar import Calendar, CalendarEvent, CalendarShare
 from app.models.user import User
+from app.services.events import notify_calendar_change
+
+
+def _cal_recipients(cal: 'Calendar'):
+    return [s.shared_with_id for s in
+            CalendarShare.query.filter_by(calendar_id=cal.id).all()]
 
 from . import dav_bp
 
@@ -513,6 +519,8 @@ def put_event(username, cal_part, filename):
     existing.ical_data = _extract_vevent_block(raw)
     existing.updated_at = datetime.now(timezone.utc)
     db.session.commit()
+    notify_calendar_change(cal.owner_id, cal.id, 'event',
+                           shared_with=_cal_recipients(cal))
 
     status = 201 if request.method == 'PUT' and not if_match else 204
     return Response('', status, {'ETag': _etag_for_event(existing)})
@@ -541,6 +549,8 @@ def delete_event(username, cal_part, filename):
         return Response('', 404)
     db.session.delete(ev)
     db.session.commit()
+    notify_calendar_change(cal.owner_id, cal.id, 'event',
+                           shared_with=_cal_recipients(cal))
     return Response('', 204)
 
 
@@ -558,8 +568,12 @@ def delete_calendar(username, cal_part):
     cal = _calendar_for(user, cal_id) if cal_id else None
     if not cal:
         return Response('', 404)
+    recipients = _cal_recipients(cal)
+    owner_id = cal.owner_id
+    cid = cal.id
     db.session.delete(cal)
     db.session.commit()
+    notify_calendar_change(owner_id, cid, 'deleted', shared_with=recipients)
     return Response('', 204)