Show agenda + notify-me (premium feature #8) — last of the 8
DB: shows (site_id, date, time, city, venue, country, ticket_url, notes).
routes/shows.js (public, premium-gated): GET /shows (upcoming shows + notify
form), POST /shows/notify (subscribers source 'notify', double opt-in if SMTP;
confirm/unsub via the generic /nieuwsbrief links).
routes/admin-shows.js (site admin + premium): GET /admin/shows (list + add
form + subscriber count), POST / (add; optional notify email to
'notify' subscribers if SMTP is available), POST /:id/delete.