feat: Kalender - Autocomplete + Privat-Flag + Share-Liste + Bugfix

Sharing-Fix:
Calendar-Model hatte keine owner-Relation zu User - list_calendars
stuerzte beim Listen geteilter Kalender ab (c.owner.username ->
AttributeError). Jetzt mit explizitem foreign_keys Relationship.

Benutzer-Autocomplete:
"Kalender teilen" nutzt jetzt /users/search wie bei Dateien.
Tippt man 2+ Zeichen, erscheint ein Dropdown mit passenden
Benutzernamen. Klick uebernimmt den Namen.

Bestehende Freigaben werden im Menue angezeigt mit Muelleimer
zum Entfernen.

Privat-Flag fuer Termine:
CalendarEvent bekommt is_private-Spalte. Checkbox im Termin-
Dialog "🔒 Privat (Teilnehmer sehen nur den Zeitblock)".

Redaction greift an drei Stellen:
* GET /events: Nicht-Owner sehen summary="Privat", description
  und location = null. Zeitfenster bleibt voll sichtbar.
* iCal-Export (/ical/<token>): Privat-Events werden mit
  CLASS:PRIVATE ausgegeben und SUMMARY/DESCRIPTION/LOCATION
  werden gestrippt.
* CalDAV: aktuell werden eh nur eigene Kalender exportiert,
  also keine Redaction noetig. Kommt bei Share-Support rein.

Der Eigentuemer sieht natuerlich in seiner eigenen Ansicht alle
Details seines privaten Termins.

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

backend/app/api/calendar.py

@@ -11,6 +11,52 @@ from app.models.calendar import Calendar, CalendarEvent, CalendarShare
 from app.models.user import User
 
 
+def _redact_if_private(event_dict: dict, is_owner: bool) -> dict:
+    """For shared viewers, strip summary/description/location from private
+    events so only the time slot remains visible."""
+    if is_owner or not event_dict.get('is_private'):
+        return event_dict
+    d = dict(event_dict)
+    d['summary'] = 'Privat'
+    d['description'] = None
+    d['location'] = None
+    return d
+
+
+def _redact_vevent(raw: str) -> str:
+    """Strip SUMMARY/DESCRIPTION/LOCATION from a VEVENT block and set
+    CLASS:PRIVATE. Used for shared iCal exports and CalDAV responses."""
+    if not raw:
+        return raw
+    import re as _re
+    out_lines = []
+    has_class = False
+    for line in raw.split('\n'):
+        stripped = line.rstrip('\r')
+        upper = stripped.split(':', 1)[0].split(';', 1)[0].upper()
+        if upper == 'SUMMARY':
+            out_lines.append('SUMMARY:Privat')
+        elif upper in ('DESCRIPTION', 'LOCATION'):
+            continue
+        elif upper == 'CLASS':
+            has_class = True
+            out_lines.append('CLASS:PRIVATE')
+        else:
+            out_lines.append(stripped)
+    if not has_class:
+        # Inject CLASS right after UID if possible, else before END:VEVENT
+        for i, l in enumerate(out_lines):
+            if l.startswith('UID:'):
+                out_lines.insert(i + 1, 'CLASS:PRIVATE')
+                break
+        else:
+            for i, l in enumerate(out_lines):
+                if l.upper().startswith('END:VEVENT'):
+                    out_lines.insert(i, 'CLASS:PRIVATE')
+                    break
+    return '\r\n'.join(out_lines)
+
+
 def _get_calendar_or_err(cal_id, user, need_write=False):
     cal = db.session.get(Calendar, cal_id)
     if not cal:
@@ -136,7 +182,8 @@ def list_events(cal_id):
             pass
 
     events = query.order_by(CalendarEvent.dtstart).all()
-    return jsonify([e.to_dict() for e in events]), 200
+    is_owner = (cal.owner_id == user.id)
+    return jsonify([_redact_if_private(e.to_dict(), is_owner) for e in events]), 200
 
 
 @api_bp.route('/calendars/<int:cal_id>/events', methods=['POST'])
@@ -184,6 +231,7 @@ def create_event(cal_id):
         dtend=dtend_dt,
         all_day=all_day,
         recurrence_rule=rrule or None,
+        is_private=bool(data.get('is_private', False)),
     )
     db.session.add(event)
     db.session.commit()
@@ -217,6 +265,8 @@ def update_event(event_id):
         event.all_day = data['all_day']
     if 'recurrence_rule' in data:
         event.recurrence_rule = (data['recurrence_rule'] or '').strip() or None
+    if 'is_private' in data:
+        event.is_private = bool(data['is_private'])
     if 'calendar_id' in data:
         new_cal, cerr = _get_calendar_or_err(data['calendar_id'], user, need_write=True)
         if cerr:
@@ -477,7 +527,10 @@ def ical_export(token):
     ]
     for e in events:
         if e.ical_data:
-            lines.append(e.ical_data)
+            block = _redact_vevent(e.ical_data) if e.is_private else e.ical_data
+            lines.append(block)
+        elif e.is_private:
+            lines.append(_build_vevent(e.uid, 'Privat', e.dtstart, e.dtend, e.all_day))
         else:
             lines.append(_build_vevent(e.uid, e.summary, e.dtstart, e.dtend, e.all_day))
     lines.append('END:VCALENDAR')