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/slots.html

@@ -32,7 +32,7 @@
 {{end}}
 </div>
 
-<div class="section-label">Going ({{.TotalGoing}})</div>
+<div class="section-label">Going ({{.TotalGoing}}{{if .Capped}} / {{.Event.AttendeeCap}}{{end}})</div>
 <div class="rsvp-list">
   {{if .GoingList}}
     {{range .GoingList}}