Add cancel RSVP feature (backend DELETE endpoint + frontend UI)

Allows guests to cancel their RSVP via a DELETE endpoint using their
guestToken. Frontend shows cancel button in RsvpBar and clears local
storage on success. Includes unit tests, integration tests, and E2E spec.

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

frontend/src/components/RsvpBar.vue

@@ -2,9 +2,32 @@
   <div class="rsvp-bar">
     <div class="rsvp-bar__inner">
       <!-- Status state: already RSVPed -->
-      <div v-if="hasRsvp" class="rsvp-bar__status">
-        <span class="rsvp-bar__check" aria-hidden="true">✓</span>
-        <span class="rsvp-bar__text">You're attending!</span>
+      <div v-if="hasRsvp" class="rsvp-bar__status-wrapper">
+        <div
+          class="rsvp-bar__status"
+          role="button"
+          tabindex="0"
+          :aria-expanded="expanded"
+          aria-label="You're attending. Tap to show cancel option."
+          @click="expanded = !expanded"
+          @keydown.enter.prevent="expanded = !expanded"
+          @keydown.space.prevent="expanded = !expanded"
+          @keydown.escape="expanded = false"
+        >
+          <span class="rsvp-bar__check" aria-hidden="true">✓</span>
+          <span class="rsvp-bar__text">You're attending!</span>
+          <span class="rsvp-bar__chevron" :class="{ 'rsvp-bar__chevron--open': expanded }" aria-hidden="true">›</span>
+        </div>
+        <Transition name="rsvp-bar-cancel">
+          <button
+            v-if="expanded"
+            class="rsvp-bar__cancel"
+            type="button"
+            @click="$emit('cancel')"
+          >
+            Cancel attendance
+          </button>
+        </Transition>
       </div>
 
       <!-- CTA state: no RSVP yet -->
@@ -18,13 +41,37 @@
 </template>
 
 <script setup lang="ts">
-defineProps<{
+import { ref, watch } from 'vue'
+
+const props = defineProps<{
   hasRsvp?: boolean
 }>()
 
 defineEmits<{
   open: []
+  cancel: []
 }>()
+
+const expanded = ref(false)
+
+watch(() => props.hasRsvp, () => {
+  expanded.value = false
+})
+
+function onClickOutside(e: MouseEvent) {
+  const target = e.target as HTMLElement
+  if (!target.closest('.rsvp-bar__status-wrapper')) {
+    expanded.value = false
+  }
+}
+
+watch(expanded, (isExpanded) => {
+  if (isExpanded) {
+    document.addEventListener('click', onClickOutside, { capture: true })
+  } else {
+    document.removeEventListener('click', onClickOutside, { capture: true })
+  }
+})
 </script>
 
 <style scoped>
@@ -73,6 +120,12 @@ defineEmits<{
   cursor: pointer;
 }
 
+.rsvp-bar__status-wrapper {
+  display: flex;
+  flex-direction: column;
+  gap: var(--spacing-xs);
+}
+
 .rsvp-bar__status {
   display: flex;
   align-items: center;
@@ -88,6 +141,13 @@ defineEmits<{
   font-weight: 600;
   font-size: 0.95rem;
   color: var(--color-text-on-gradient);
+  cursor: pointer;
+  user-select: none;
+  -webkit-user-select: none;
+}
+
+.rsvp-bar__status:hover {
+  background: linear-gradient(135deg, var(--color-glass-hover) 0%, var(--color-glass-strong) 100%);
 }
 
 .rsvp-bar__check {
@@ -101,4 +161,49 @@ defineEmits<{
   text-overflow: ellipsis;
   white-space: nowrap;
 }
+
+.rsvp-bar__chevron {
+  font-size: 1.2rem;
+  font-weight: 700;
+  transition: transform 0.2s ease;
+  transform: rotate(0deg);
+  margin-left: auto;
+}
+
+.rsvp-bar__chevron--open {
+  transform: rotate(90deg);
+}
+
+.rsvp-bar__cancel {
+  display: block;
+  width: 100%;
+  padding: var(--spacing-sm) var(--spacing-lg);
+  border-radius: var(--radius-card);
+  font-family: inherit;
+  font-size: 0.9rem;
+  font-weight: 600;
+  color: #ef5350;
+  background: linear-gradient(135deg, var(--color-glass-strong) 0%, var(--color-glass-subtle) 100%);
+  border: 1px solid var(--color-glass-border);
+  backdrop-filter: blur(16px);
+  -webkit-backdrop-filter: blur(16px);
+  cursor: pointer;
+  text-align: center;
+  transition: background 0.15s ease;
+}
+
+.rsvp-bar__cancel:hover {
+  background: linear-gradient(135deg, var(--color-glass-hover) 0%, var(--color-glass-strong) 100%);
+}
+
+.rsvp-bar-cancel-enter-active,
+.rsvp-bar-cancel-leave-active {
+  transition: opacity 0.15s ease, transform 0.15s ease;
+}
+
+.rsvp-bar-cancel-enter-from,
+.rsvp-bar-cancel-leave-to {
+  opacity: 0;
+  transform: translateY(-4px);
+}
 </style>