feat(admin-events): expose membersOnly toggle in the form

The Event model and Zod schemas already supported membersOnly, but the
admin form never exposed it — public/private was implicit and not
editable from the UI.

Add a fifth checkbox alongside the other Event Settings, hydrate it on
edit, reset it in saveAndCreateAnother.

app/pages/admin/events/create.vue

@@ -536,6 +536,16 @@
                   <span class="help-text"> Mark this event as cancelled </span>
                 </div>
               </label>
+
+              <label class="check-label">
+                <input v-model="eventForm.membersOnly" type="checkbox" />
+                <div>
+                  <strong>Members Only</strong>
+                  <span class="help-text">
+                    Hide this event from the public; only members can see it
+                  </span>
+                </div>
+              </label>
             </div>
           </div>
         </div>
@@ -622,6 +632,7 @@ const eventForm = reactive({
   isOnline: true,
   isVisible: true,
   isCancelled: false,
+  membersOnly: false,
   cancellationMessage: "",
   targetCircles: [],
   tags: [],
@@ -724,6 +735,7 @@ function populateEditForm(payload) {
     isOnline: event.isOnline,
     isVisible: event.isVisible !== undefined ? event.isVisible : true,
     isCancelled: event.isCancelled || false,
+    membersOnly: event.membersOnly || false,
     cancellationMessage: event.cancellationMessage || "",
     targetCircles: event.targetCircles || [],
     tags: event.tags || [],
@@ -950,6 +962,7 @@ const saveAndCreateAnother = async () => {
       isOnline: true,
       isVisible: true,
       isCancelled: false,
+      membersOnly: false,
       cancellationMessage: "",
       targetCircles: [],
       tags: [],