Add yes/no RSVP answers with required login

RSVPs now carry a status ('yes'/'no'). When auth is enabled, answering
requires login for all events, not just capped ones. Answers are deduped
one per user and re-answering updates in place. A 'no' zeroes plus_one,
is excluded from Going counts and cap math, frees the person's slot
claims, and shows in a separate "Can't make it" list.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

templates/layout.html

@@ -286,6 +286,13 @@
     box-shadow: 4px 4px 0 var(--sage);
     transform: translate(-2px, -2px);
   }
+  .btn-no {
+    background: var(--cream);
+    color: var(--ink);
+  }
+  .btn-no:hover {
+    box-shadow: 4px 4px 0 var(--peach);
+  }
   .progress-bar {
     height: 4px;
     background: #e8e8e8;