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/home.html

@@ -33,6 +33,12 @@
       <label>Description (optional, markdown supported)</label>
       <textarea name="description" rows="4" placeholder="e.g. Bring your own blanket! We'll have a grill set up near the big oak tree." style="border:var(--border-w) solid var(--border);background:var(--cream);padding:10px 14px;font-family:'Bricolage Grotesque',sans-serif;font-size:0.95rem;resize:vertical;outline:none;"></textarea>
     </div>
+    {{if .AuthEnabled}}
+    <div class="form-row">
+      <label>Attendee cap (optional — guests must log in to RSVP)</label>
+      <input type="number" name="attendee_cap" min="0" max="1000" placeholder="e.g. 30 — leave blank for unlimited">
+    </div>
+    {{end}}
 
     <div class="section-label" style="margin:24px 0 16px">Slots</div>