Add a lightweight schema migration runner, fix missing PHPMailer in prod image

docker-entrypoint-initdb.d only applies db/init.sql, and only on a brand new
volume - existing installs never got db/migrations/*.sql applied unless
someone ran them by hand. This bit twice this session (qr-oss's own init.sql
was missing a migration, and qr-vip's live DB was two migrations behind after
a code deploy). scripts/migrate.php now runs automatically on every container
start via a new entrypoint wrapper, applying any not-yet-recorded migration -
safe to run repeatedly since every migration file already checks
information_schema before altering.

Also caught in the process: Dockerfile.fpm (the production image) never
installed phpmailer/phpmailer, unlike the dev Dockerfile - meaning
self-registration's password email has been silently fatal-erroring in
production since the nginx+php-fpm switch.

Dockerfile

@@ -97,11 +97,20 @@ WORKDIR /var/www/html
 RUN composer update
 RUN composer require phpmailer/phpmailer:^6.9
 COPY ./src ./
+COPY ./db/migrations ./db/migrations
 RUN chmod 755 *;
 
 # Qr code storage lives outside the document root so files can only be reached through
 # the authenticated qrcode_image.php / qrcode_zip_download.php endpoints.
 RUN mkdir -p /var/www/qrcode-storage/zip && chmod -R 777 /var/www/qrcode-storage
 
+# Applies any not-yet-applied db/migrations/*.sql on every container start (see
+# src/scripts/migrate.php) - docker-entrypoint-initdb.d only runs db/init.sql, and only
+# on a brand new volume, so without this an existing install's schema silently falls
+# behind the code on every `git pull` + restart.
+COPY docker/entrypoint.sh /usr/local/bin/entrypoint.sh
+RUN chmod +x /usr/local/bin/entrypoint.sh
+ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]
+
 EXPOSE 80
 CMD ["php", "-S", "0.0.0.0:80"]