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/login.html

@@ -14,6 +14,7 @@
 <div class="claim-form-wrapper">
   {{if eq .Step "identify"}}
   <form method="POST" action="/login">
+    {{if .Next}}<input type="hidden" name="next" value="{{.Next}}">{{end}}
     <div class="form-row">
       <label>Email</label>
       <input type="email" name="identifier" placeholder="you@example.com" required autofocus>
@@ -23,6 +24,7 @@
   {{else}}
   <form method="POST" action="/login/verify">
     <input type="hidden" name="identifier" value="{{.Identifier}}">
+    {{if .Next}}<input type="hidden" name="next" value="{{.Next}}">{{end}}
     <p style="font-family:'DM Mono',monospace;font-size:0.78rem;color:#555;margin-bottom:16px;">
       Code sent to <strong>{{.Identifier}}</strong>
     </p>