calendar: add category-group links, fix iCal share URL, remove legend

- Link categories to user groups via new zielgruppen column on
  veranstaltung_kategorien (migration 017), editable in the category
  management UI with group checkboxes and chip display
- Fix broken iCal share link by adding ICAL_BASE_URL to docker-compose
  and falling back to CORS_ORIGIN when ICAL_BASE_URL is unset
- Remove the colored-dot legend footer from the month calendar view

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/services/booking.service.ts

@@ -310,7 +310,7 @@ class BookingService {
       logger.info('Created new iCal token for user', { userId });
     }
 
-    const baseUrl = process.env.ICAL_BASE_URL || 'http://localhost:3000';
+    const baseUrl = (process.env.ICAL_BASE_URL || process.env.CORS_ORIGIN || 'http://localhost:3000').replace(/\/$/, '');
     const subscribeUrl = `${baseUrl}/api/bookings/calendar.ics?token=${token}`;
     return { token, subscribeUrl };
   }