feat: Persoenliche Farbe fuer freigegebene Kalender

CalendarShare bekommt color-Spalte. Im Kalender-Menue kann jeder
Benutzer eine eigene Anzeigefarbe fuer einen mit ihm geteilten
Kalender setzen, ohne dass sich dadurch die Farbe beim
Eigentuemer oder anderen Share-Empfaengern aendert.

* Owner: Farbe aendert den Kalender direkt (wie bisher).
* Share-Empfaenger: Farbe landet in CalendarShare.color und wird
  nur fuer ihn ausgeliefert (list_calendars injiziert sie in
  'color', Owner-Farbe bleibt in 'owner_color' als Referenz).

Neuer Endpoint: PUT /calendars/<id>/my-color.
UI-Hinweis: "Nur fuer deine Ansicht - <Owner> behaelt seine Farbe".

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

backend/app/api/calendar.py

@@ -100,6 +100,11 @@ def list_calendars():
             calendar_id=c.id, shared_with_id=user.id
         ).first()
         d['permission'] = share.permission if share else 'read'
+        # Per-user color override: the owner's color is kept in 'owner_color'
+        # so the UI can show both, and 'color' reflects what this user picked.
+        d['owner_color'] = c.color
+        if share and share.color:
+            d['color'] = share.color
         d['owner_name'] = c.owner.username
         result.append(d)
 
@@ -146,6 +151,33 @@ def update_calendar(cal_id):
     return jsonify(cal.to_dict()), 200
 
 
+@api_bp.route('/calendars/<int:cal_id>/my-color', methods=['PUT'])
+@token_required
+def set_my_calendar_color(cal_id):
+    """Personal display color for a shared calendar. Doesn't affect the
+    owner's calendar color or any other user's view."""
+    user = request.current_user
+    cal = db.session.get(Calendar, cal_id)
+    if not cal:
+        return jsonify({'error': 'Nicht gefunden'}), 404
+
+    color = (request.get_json() or {}).get('color', '').strip()
+
+    if cal.owner_id == user.id:
+        # Owner -> update the calendar itself
+        if color:
+            cal.color = color
+        db.session.commit()
+        return jsonify({'color': cal.color}), 200
+
+    share = CalendarShare.query.filter_by(calendar_id=cal_id, shared_with_id=user.id).first()
+    if not share:
+        return jsonify({'error': 'Kein Zugriff'}), 403
+    share.color = color or None
+    db.session.commit()
+    return jsonify({'color': share.color or cal.color}), 200
+
+
 @api_bp.route('/calendars/<int:cal_id>', methods=['DELETE'])
 @token_required
 def delete_calendar(cal_id):

backend/app/models/calendar.py

@@ -82,6 +82,7 @@ class CalendarShare(db.Model):
     calendar_id = db.Column(db.Integer, db.ForeignKey('calendars.id'), nullable=False, index=True)
     shared_with_id = db.Column(db.Integer, db.ForeignKey('users.id'), nullable=False, index=True)
     permission = db.Column(db.String(20), nullable=False, default='read')  # 'read' or 'readwrite'
+    color = db.Column(db.String(7), nullable=True)  # Persoenliche Anzeige-Farbe
 
     shared_with = db.relationship('User', backref='shared_calendars')