Add birthdate handling in member registration and management. Update UI to conditionally require birthdate for new members, and enhance API to enforce birthdate validation. Improve tests to cover new birthdate requirements.

components/cms/CmsMitglieder.vue

@@ -469,12 +469,12 @@
             <input
               v-model="formData.geburtsdatum"
               type="date"
-              required
+              :required="isBirthdateRequired"
               class="w-full px-4 py-2 border border-gray-300 rounded-lg focus:outline-none focus:ring-2 focus:ring-primary-500"
               :disabled="isSaving"
             >
             <p class="text-xs text-gray-500 mt-1">
-              Wird zur eindeutigen Identifizierung benötigt
+              Fuer neue Mitglieder erforderlich. Altdaten ohne Geburtsdatum koennen weiter bearbeitet werden.
             </p>
           </div>
 
@@ -811,6 +811,10 @@ const canViewContactData = computed(() => {
   return authStore.hasRole('vorstand')
 })
 
+const isBirthdateRequired = computed(() => {
+  return !editingMember.value || Boolean(editingMember.value?.geburtsdatum)
+})
+
 const filteredMembers = computed(() => {
   if (!filterHasHallKey.value) return members.value
   return members.value.filter(member => member.hasHallKey)