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/database/migrations/017_add_kategorie_zielgruppen.sql

@@ -0,0 +1,7 @@
+-- Migration 017: Add zielgruppen (target groups) to event categories
+-- Links categories to user groups for visibility filtering
+ALTER TABLE veranstaltung_kategorien
+    ADD COLUMN IF NOT EXISTS zielgruppen TEXT[] NOT NULL DEFAULT '{}';
+
+-- Comment for documentation
+COMMENT ON COLUMN veranstaltung_kategorien.zielgruppen IS 'Array of Authentik group names this category is linked to';