Adds a mailpit service (axllent/mailpit, ~15 MB, in-memory) to
docker-compose.yml — chosen over the unmaintained MailHog for the same
footprint. It has no profile, so `docker compose up -d` starts it alongside
the API; the app defaults to MAIL_TRANSPORT=smtp -> mailpit:1025 (no
auth/TLS) and mail is read at http://localhost:8025.
Also change the default MAIL_FROM to no-reply@todo.test: PHPMailer v7 rejects
the dotless no-reply@localhost as an invalid address.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>