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.

README.md

@@ -43,7 +43,8 @@ project by Giandonato Inverso, built on [AdminLTE](https://adminlte.io/).
 # What is included
 
 - PHP 8.4 application source
-- Database schema + migrations
+- Database schema + migrations (applied automatically on every container start,
+  so `git pull` + restart is enough to bring an existing install up to date)
 - Docker Compose files (dev and production)
 - CSS/JS assets