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/services/events.ts

@@ -37,6 +37,7 @@ export const eventsApi = {
     beschreibung?: string;
     farbe?: string;
     icon?: string;
+    zielgruppen?: string[];
   }): Promise<VeranstaltungKategorie> {
     return api
       .post<ApiResponse<VeranstaltungKategorie>>('/api/events/kategorien', data)
@@ -46,7 +47,7 @@ export const eventsApi = {
   /** Update an existing event category */
   updateKategorie(
     id: string,
-    data: Partial<{ name: string; beschreibung?: string; farbe?: string; icon?: string }>
+    data: Partial<{ name: string; beschreibung?: string; farbe?: string; icon?: string; zielgruppen?: string[] }>
   ): Promise<VeranstaltungKategorie> {
     return api
       .patch<ApiResponse<VeranstaltungKategorie>>(`/api/events/kategorien/${id}`, data)