Make expiryDate an internal concern, auto-set to event date + 7 days

The expiry date is no longer user-facing: it is removed from the API
(request and response) and the frontend. The backend now automatically
calculates it as the event date plus 7 days. The expired banner and
RSVP-bar filtering by expired status are also removed from the UI,
since expiry is purely an internal data-retention mechanism.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/views/EventDetailView.vue

@@ -25,10 +25,6 @@
 
       <!-- Loaded state -->
       <div v-else-if="state === 'loaded' && event" class="detail__content">
-        <div v-if="event.expired" class="detail__banner detail__banner--expired" role="status">
-          This event has ended.
-        </div>
-
         <h1 class="detail__title">{{ event.title }}</h1>
 
         <dl class="detail__meta">
@@ -74,9 +70,9 @@
       </div>
     </div>
 
-    <!-- RSVP bar (only for loaded, non-expired events) -->
+    <!-- RSVP bar -->
     <RsvpBar
-      v-if="state === 'loaded' && event && !event.expired && !isOrganizer"
+      v-if="state === 'loaded' && event && !isOrganizer"
       :has-rsvp="!!rsvpName"
       @open="sheetOpen = true"
     />
@@ -412,12 +408,6 @@ onMounted(fetchEvent)
   text-align: center;
 }
 
-.detail__banner--expired {
-  background: var(--color-glass);
-  color: var(--color-text-soft);
-  backdrop-filter: blur(4px);
-}
-
 /* Error / not-found message */
 .detail__message {
   font-size: 1.1rem;