Add edit RSVP modal, plus-one tracking, and unified signup form

Merge RSVP + slot claim into a single form. Add plus_one field to RSVPs.
Add clickable RSVP names that open a modal to edit name/note/plus_one.

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

templates/edit-rsvp.html

@@ -0,0 +1,27 @@
+{{define "edit-rsvp"}}
+<div class="modal-backdrop" onclick="if(event.target===this)document.getElementById('edit-modal').innerHTML=''">
+  <div class="modal-card">
+    <div class="form-title">Edit RSVP</div>
+    <form hx-put="/e/{{.Slug}}/rsvp/{{.Rsvp.ID}}"
+          hx-target="#slots-container"
+          hx-swap="innerHTML settle:0.1s">
+      <div class="form-row">
+        <label>Your name</label>
+        <input type="text" name="name" value="{{.Rsvp.Name}}" required>
+      </div>
+      <div class="form-row">
+        <label>Bringing anyone?</label>
+        <input type="number" name="plus_one" value="{{.Rsvp.PlusOne}}" min="0" max="10">
+      </div>
+      <div class="form-row">
+        <label>Note (optional)</label>
+        <input type="text" name="note" value="{{.Rsvp.Note}}">
+      </div>
+      <button class="btn-submit" type="submit">Save</button>
+      <div style="text-align:center;margin-top:12px;">
+        <button type="button" class="btn-cancel" onclick="document.getElementById('edit-modal').innerHTML=''">Cancel</button>
+      </div>
+    </form>
+  </div>
+</div>
+{{end}}