Add RSVP support and Open Graph meta tags

People can now RSVP without claiming a slot. OG tags show
event title, date/time/location, and headcount in link previews.

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

templates/slots.html

@@ -1,4 +1,6 @@
 {{define "slots-inner"}}
+<div class="section-label">What's needed</div>
+<div class="slots-grid">
 {{range .Slots}}
 <div class="slot-card{{if .IsFull}} full{{end}}">
   <div class="slot-info">
@@ -28,6 +30,25 @@
   </div>
 </div>
 {{end}}
+</div>
+
+<div class="section-label">Going ({{len .Rsvps}})</div>
+<div class="rsvp-list">
+  {{if .Rsvps}}
+    {{range .Rsvps}}
+    <span class="claim-chip">
+      {{.Name}}{{if .Note}} <small style="color:#888">({{.Note}})</small>{{end}}
+      <button hx-delete="/e/{{$.Event.Slug}}/rsvp/{{.ID}}"
+              hx-target="#slots-container"
+              hx-swap="innerHTML settle:0.1s"
+              hx-confirm="Remove {{.Name}}?"
+              title="Remove">&#215;</button>
+    </span>
+    {{end}}
+  {{else}}
+    <span class="nobody">no one yet</span>
+  {{end}}
+</div>
 {{end}}
 
 {{define "slots.html"}}{{template "slots-inner" .}}{{end}}