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>

frontend/src/types/events.types.ts

@@ -8,6 +8,7 @@ export interface VeranstaltungKategorie {
   beschreibung?: string | null;
   farbe: string; // hex color e.g. '#1976d2'
   icon?: string | null; // MUI icon name
+  zielgruppen: string[];
   erstellt_am: string;
   aktualisiert_am: string;
 }