fix: CalDAV HEAD auf Events + PROPPATCH auf Kalender

* GET-Route akzeptiert jetzt auch HEAD - manche Clients pruefen
  Existenz einer Ressource via HEAD bevor sie GET senden.
* Neue PROPPATCH-Route auf der Kalender-Collection: erkennt
  calendar-color + displayname und persistiert beides. Andere
  Properties werden als "angewendet" bestaetigt, damit DAVx5
  und Apple Calendar nicht enttaeuscht sind.

Damit sollten die 500-Fehler beim Sync verschwinden. Falls nicht,
bitte Server- oder DAVx5-Log posten.

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

backend/app/dav/caldav.py

@@ -386,7 +386,7 @@ def _extract_time_range(root: ET.Element):
 # GET single event
 # ---------------------------------------------------------------------------
 
-@dav_bp.route('/<username>/<cal_part>/<filename>', methods=['GET'])
+@dav_bp.route('/<username>/<cal_part>/<filename>', methods=['GET', 'HEAD'])
 @basic_auth
 def get_event(username, cal_part, filename):
     user: User = request.dav_user
@@ -500,6 +500,53 @@ def delete_calendar(username, cal_part):
     return Response('', 204)
 
 
+# ---------------------------------------------------------------------------
+# PROPPATCH (Clients setzen gerne Anzeigefarbe/-name). Wir persistieren
+# den Kalenderfarbe (calendar-color) + Displayname; andere Properties
+# bestaetigen wir als "angewendet" damit DAVx5/Apple zufrieden sind.
+# ---------------------------------------------------------------------------
+
+@dav_bp.route('/<username>/<cal_part>/', methods=['PROPPATCH'])
+@dav_bp.route('/<username>/<cal_part>', methods=['PROPPATCH'])
+@basic_auth
+def proppatch_calendar(username, cal_part):
+    user: User = request.dav_user
+    if username != user.username:
+        return Response('', 403)
+    cal_id = _parse_calendar_path(cal_part)
+    cal = _calendar_for(user, cal_id) if cal_id else None
+    if not cal:
+        return Response('Not found', 404)
+    try:
+        root = ET.fromstring(request.data or b'<x/>')
+    except ET.ParseError:
+        return Response('Malformed XML', 400)
+
+    for el in root.iter():
+        tag = el.tag
+        if tag == _qn('ic', 'calendar-color') and el.text:
+            cal.color = el.text.strip()[:7]
+        elif tag == _qn('d', 'displayname') and el.text:
+            cal.name = el.text.strip()[:255]
+    db.session.commit()
+
+    # Respond with 207 marking everything as applied so the client is happy.
+    multistatus = ET.Element(_qn('d', 'multistatus'))
+    href = _href_calendar(user.username, cal.id)
+    resp = ET.SubElement(multistatus, _qn('d', 'response'))
+    ET.SubElement(resp, _qn('d', 'href')).text = href
+    propstat = ET.SubElement(resp, _qn('d', 'propstat'))
+    prop = ET.SubElement(propstat, _qn('d', 'prop'))
+    # Echo back everything the client asked to set
+    for set_block in root.findall(_qn('d', 'set')):
+        inner_prop = set_block.find(_qn('d', 'prop'))
+        if inner_prop is not None:
+            for child in inner_prop:
+                ET.SubElement(prop, child.tag)
+    ET.SubElement(propstat, _qn('d', 'status')).text = 'HTTP/1.1 200 OK'
+    return _xml_response(multistatus)
+
+
 # ---------------------------------------------------------------------------
 # MKCALENDAR (create a new calendar collection via the DAV URL)
 # ---------------------------------------------------------------------------