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/edit-rsvp.html

@@ -9,6 +9,13 @@
         <label>Your name</label>
         <input type="text" name="name" value="{{.Rsvp.Name}}" required>
       </div>
+      <div class="form-row">
+        <label>Coming?</label>
+        <select name="status">
+          <option value="yes" {{if ne .Rsvp.Status "no"}}selected{{end}}>Yes, I'm going</option>
+          <option value="no" {{if eq .Rsvp.Status "no"}}selected{{end}}>Can't make it</option>
+        </select>
+      </div>
       <div class="form-row">
         <label>Bringing anyone?</label>
         <input type="number" name="plus_one" value="{{.Rsvp.PlusOne}}" min="0" max="10">