feat(auth): Add Mailpit (fake SMTP) to docker-compose for local email

[ebouchut]

Add a Mailpit fake-SMTP catcher for local development so the password-reset email can be exercised end-to-end without a real provider.

• Add a mailpit service to docker-compose.yaml (SMTP 1025, web UI 8025).
• Configure spring.mail.* in the dev profile (host: localhost, port: 1025, no auth/STARTTLS).
• Document the UI URL in README/CONTRIBUTING.

Chosen over MailHog (abandoned since 2020); Spring config is identical either way.