Add hotel registration with email confirmation (nodemailer, confirm-email endpoint)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/005_email_confirmation.sql

@@ -0,0 +1,7 @@
+ALTER TABLE users
+  ADD COLUMN IF NOT EXISTS email_confirmed      BOOLEAN      NOT NULL DEFAULT FALSE,
+  ADD COLUMN IF NOT EXISTS confirmation_token   VARCHAR(255),
+  ADD COLUMN IF NOT EXISTS confirmation_sent_at TIMESTAMPTZ;
+
+-- Existing users (seed data) are already confirmed
+UPDATE users SET email_confirmed = TRUE WHERE confirmation_token IS NULL;