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>

db/queries.sql

@@ -5,8 +5,8 @@ SELECT * FROM events WHERE slug = ?;
 SELECT * FROM events WHERE admin_token = ?;
 
 -- name: CreateEvent :one
-INSERT INTO events (slug, title, date, time, location, admin_token, description)
-VALUES (?, ?, ?, ?, ?, ?, ?)
+INSERT INTO events (slug, title, date, time, location, admin_token, description, attendee_cap)
+VALUES (?, ?, ?, ?, ?, ?, ?, ?)
 RETURNING *;
 
 -- name: UpdateEventDescription :exec
@@ -65,8 +65,8 @@ SELECT COUNT(*) FROM claims WHERE slot_id = ?;
 SELECT * FROM rsvps WHERE event_id = ? ORDER BY created_at;
 
 -- name: CreateRsvp :one
-INSERT INTO rsvps (event_id, name, note, plus_one)
-VALUES (?, ?, ?, ?)
+INSERT INTO rsvps (event_id, name, note, plus_one, user_id)
+VALUES (?, ?, ?, ?, ?)
 RETURNING *;
 
 -- name: DeleteRsvp :exec
@@ -78,6 +78,12 @@ SELECT COUNT(*) FROM rsvps WHERE event_id = ?;
 -- name: GetRsvpByName :one
 SELECT * FROM rsvps WHERE event_id = ? AND name = ? COLLATE NOCASE LIMIT 1;
 
+-- name: GetRsvpByUser :one
+SELECT * FROM rsvps WHERE event_id = ? AND user_id = ? LIMIT 1;
+
+-- name: CountGoing :one
+SELECT CAST(COALESCE(SUM(1 + plus_one), 0) AS INTEGER) FROM rsvps WHERE event_id = ?;
+
 -- name: DeleteDuplicateRsvps :exec
 DELETE FROM rsvps WHERE id NOT IN (
   SELECT MIN(id) FROM rsvps GROUP BY event_id, name COLLATE NOCASE