Bundle a Mailpit catcher and default Compose to SMTP

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>

src/Support/Config.php

@@ -50,7 +50,7 @@ final class Config
 
         $mail = new MailConfig(
             transport: strtolower(self::env('MAIL_TRANSPORT', 'mail')),
-            fromAddress: self::env('MAIL_FROM', 'no-reply@localhost'),
+            fromAddress: self::env('MAIL_FROM', 'no-reply@todo.test'),
             fromName: self::env('MAIL_FROM_NAME', 'Todo List'),
             logPath: $mailLogPath,
             smtpHost: self::env('MAIL_SMTP_HOST'),