feat: Serientermin-Bearbeitung: Nur diesen Termin oder Serie

Klick auf einen wiederkehrenden Termin oeffnet zuerst einen Dialog:
"Nur diesen Termin" oder "Ganze Serie".

* Serie: bearbeitet den Master wie bisher
* Nur dieser: fuegt EXDATE fuer das geklickte Datum zum Master
  hinzu und legt einen eigenstaendigen Ersatz-Termin mit den
  bearbeiteten Daten an

Backend:
* CalendarEvent.exdates speichert Ausnahmedaten kommasepariert
* POST /events/<id>/exception fuegt EXDATE hinzu, erstellt
  optional das Replacement-Event mit frischer UID
* _build_vevent schreibt jetzt EXDATE-Zeilen in die ical_data,
  sodass CalDAV-Clients die Ausnahmen auch sehen werden

Frontend:
* FullCalendar rrule-Plugin bekommt die exdate-Liste und blendet
  die uebersprungenen Tage aus
* Drag & Drop verschiebt weiterhin die ganze Serie (Shortcut -
  fuer Einzelverschiebung Termin anklicken und bearbeiten)

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

backend/app/api/calendar.py

@@ -171,7 +171,7 @@ def create_event(cal_id):
     rrule = (data.get('recurrence_rule') or '').strip()
 
     ical_data = _build_ical(event_uid, summary, dtstart_dt, dtend_dt, all_day,
-                            description, location, rrule)
+                            description, location, rrule, None)
 
     event = CalendarEvent(
         calendar_id=cal_id,
@@ -226,13 +226,87 @@ def update_event(event_id):
     event.ical_data = _build_ical(
         event.uid, event.summary, event.dtstart, event.dtend,
         event.all_day, event.description or '', event.location or '',
-        event.recurrence_rule or ''
+        event.recurrence_rule or '',
+        event.exdates.split(',') if event.exdates else None,
     )
     event.updated_at = datetime.now(timezone.utc)
     db.session.commit()
     return jsonify(event.to_dict()), 200
 
 
+@api_bp.route('/events/<int:event_id>/exception', methods=['POST'])
+@token_required
+def add_event_exception(event_id):
+    """Exclude a single occurrence of a recurring event ("nur dieser Termin").
+    Optionally creates a standalone replacement event for that date."""
+    user = request.current_user
+    event = db.session.get(CalendarEvent, event_id)
+    if not event:
+        return jsonify({'error': 'Event nicht gefunden'}), 404
+
+    cal, err = _get_calendar_or_err(event.calendar_id, user, need_write=True)
+    if err:
+        return err
+    if not event.recurrence_rule:
+        return jsonify({'error': 'Kein Serientermin'}), 400
+
+    data = request.get_json()
+    occurrence_date = data.get('occurrence_date')  # ISO date or datetime
+    if not occurrence_date:
+        return jsonify({'error': 'occurrence_date erforderlich'}), 400
+
+    # Normalize to YYYY-MM-DD for storage key
+    try:
+        parsed = datetime.fromisoformat(occurrence_date.replace('Z', '+00:00'))
+        key = parsed.strftime('%Y-%m-%d' if event.all_day else '%Y-%m-%dT%H:%M:%S')
+    except ValueError:
+        key = occurrence_date
+
+    existing = (event.exdates or '').split(',') if event.exdates else []
+    if key not in existing:
+        existing.append(key)
+    event.exdates = ','.join(filter(None, existing))
+
+    # Optional: create replacement single event
+    replacement = None
+    if data.get('replacement'):
+        r = data['replacement']
+        rep_uid = str(uuid.uuid4())
+        rep_start = datetime.fromisoformat(r['dtstart'])
+        rep_end = datetime.fromisoformat(r['dtend']) if r.get('dtend') else rep_start
+        replacement = CalendarEvent(
+            calendar_id=event.calendar_id,
+            uid=rep_uid,
+            summary=r.get('summary', event.summary),
+            description=r.get('description', event.description),
+            location=r.get('location', event.location),
+            dtstart=rep_start,
+            dtend=rep_end,
+            all_day=r.get('all_day', event.all_day),
+            recurrence_rule=None,
+            ical_data='',
+        )
+        replacement.ical_data = _build_ical(
+            rep_uid, replacement.summary, rep_start, rep_end,
+            replacement.all_day, replacement.description or '',
+            replacement.location or '', '',
+        )
+        db.session.add(replacement)
+
+    event.ical_data = _build_ical(
+        event.uid, event.summary, event.dtstart, event.dtend,
+        event.all_day, event.description or '', event.location or '',
+        event.recurrence_rule or '',
+        event.exdates.split(',') if event.exdates else None,
+    )
+    event.updated_at = datetime.now(timezone.utc)
+    db.session.commit()
+    return jsonify({
+        'event': event.to_dict(),
+        'replacement': replacement.to_dict() if replacement else None,
+    }), 200
+
+
 @api_bp.route('/events/<int:event_id>', methods=['DELETE'])
 @token_required
 def delete_event(event_id):
@@ -423,7 +497,7 @@ def _format_dt(dt, all_day=False):
     return dt.strftime('%Y%m%dT%H%M%SZ')
 
 
-def _build_vevent(uid, summary, dtstart, dtend, all_day, description='', location='', rrule=''):
+def _build_vevent(uid, summary, dtstart, dtend, all_day, description='', location='', rrule='', exdates=None):
     if not dtend:
         dtend = dtstart
     lines = [
@@ -443,10 +517,21 @@ def _build_vevent(uid, summary, dtstart, dtend, all_day, description='', locatio
         lines.append(f'LOCATION:{location}')
     if rrule:
         lines.append(f'RRULE:{rrule}')
+    if exdates:
+        for ex in exdates:
+            if all_day:
+                lines.append(f'EXDATE;VALUE=DATE:{ex.replace("-", "")}')
+            else:
+                # Convert ISO datetime (with or without TZ) into YYYYMMDDTHHMMSSZ
+                try:
+                    dt = datetime.fromisoformat(ex.replace('Z', '+00:00'))
+                    lines.append(f'EXDATE:{dt.strftime("%Y%m%dT%H%M%SZ")}')
+                except ValueError:
+                    pass
     lines.append(f'DTSTAMP:{datetime.now(timezone.utc).strftime("%Y%m%dT%H%M%SZ")}')
     lines.append('END:VEVENT')
     return '\r\n'.join(lines)
 
 
-def _build_ical(uid, summary, dtstart, dtend, all_day, description='', location='', rrule=''):
-    return _build_vevent(uid, summary, dtstart, dtend, all_day, description, location, rrule)
+def _build_ical(uid, summary, dtstart, dtend, all_day, description='', location='', rrule='', exdates=None):
+    return _build_vevent(uid, summary, dtstart, dtend, all_day, description, location, rrule, exdates)