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/slots.html

@@ -37,7 +37,10 @@
   {{if .GoingList}}
     {{range .GoingList}}
     <span class="claim-chip">
-      {{.Name}}{{if .Note}} <small style="color:#888">({{.Note}})</small>{{end}}
+      {{if gt .RsvpID 0}}<span class="rsvp-name-link"
+        hx-get="/e/{{$.Event.Slug}}/rsvp/{{.RsvpID}}/edit"
+        hx-target="#edit-modal"
+        hx-swap="innerHTML">{{.Name}}</span>{{else}}{{.Name}}{{end}}{{if gt .PlusOne 0}} +{{.PlusOne}}{{end}}{{if .Note}} <small style="color:#888">({{.Note}})</small>{{end}}
       {{if gt .RsvpID 0}}
       <button hx-delete="/e/{{$.Event.Slug}}/rsvp/{{.RsvpID}}"
               hx-target="#slots-container"