Created Turnpike, event attendee and volunteer management

Built after prototype, Traverse, an attendee and volunteer list
maintainer.

frontend/src/components/CheckInButton.svelte

@@ -0,0 +1,13 @@
+<script>
+  let { onclick } = $props()
+  let loading = $state(false)
+
+  async function handle() {
+    loading = true
+    try { await onclick() } finally { loading = false }
+  }
+</script>
+
+<button class="btn btn-success btn-sm" onclick={handle} disabled={loading}>
+  {loading ? '…' : '✓ Check in'}
+</button>