Add attendee cap with required login for capped events

Hosts can optionally cap total attendance (RSVPs + plus-ones) when
creating an event. Capped events require guests to log in via the
existing email code flow; RSVPs are tied to accounts and deduped per
user. The event page shows spots left and replaces the form with a
full notice when the cap is reached; the server rejects over-cap
RSVPs and edits with 409 as a race backstop.

Also threads a safeNext-validated ?next= param through the login and
name-setup flow so guests land back on the event after signing in.

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

templates/name.html

@@ -13,6 +13,7 @@
 
 <div class="claim-form-wrapper">
   <form method="POST" action="/account/name">
+    {{if .Next}}<input type="hidden" name="next" value="{{.Next}}">{{end}}
     <div class="form-row">
       <label>Name</label>
       <input type="text" name="name" placeholder="Your name" required autofocus