Simplify registration flow: mandatory signup redirect, payment emails, payment reminder cron

- EventRegistrationForm/WatcherForm/PerformerForm: remove session/login nudge/SuccessScreen;
  both roles redirect to /login?signup=1&email=<email>&next=/account after submit
- SuccessScreen.tsx deleted (no longer used)
- account.tsx: add 'Betaal nu' CTA via checkoutMutation (shown when watcher + paymentStatus pending)
- DB schema: add paymentReminderSentAt column to registration table
- Migration: 0008_payment_reminder_sent_at.sql
- email.ts: update registrationConfirmationHtml / sendConfirmationEmail with signupUrl param;
  add sendPaymentReminderEmail and sendPaymentConfirmationEmail functions
- routers/index.ts: wire payment reminder into runSendReminders() — queries watchers
  with paymentStatus pending, paymentReminderSentAt IS NULL, createdAt <= now-3days
- mollie.ts webhook: call sendPaymentConfirmationEmail after marking registration paid

packages/db/src/migrations/0008_payment_reminder_sent_at.sql

@@ -0,0 +1,2 @@
+-- Migration: add payment_reminder_sent_at column to registration table
+ALTER TABLE registration ADD COLUMN payment_reminder_sent_at INTEGER;