Add back SMTP authentication for port 2525

Restore SMTP username and password for authenticated relay on port
2525. According to Cloudron docs, this port should work with plain
SMTP and authentication without STARTTLS.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

start.sh

@@ -125,8 +125,8 @@ SESSION_SECRET_FILE="$SECRETS_DIR/session_secret"
 SMTP_HOST="${CLOUDRON_MAIL_SMTP_SERVER:-mail}"
 SMTP_PORT="${CLOUDRON_MAIL_SMTP_PORT:-2525}"
 SMTP_ENCRYPTION=""
-SMTP_USERNAME=""
-SMTP_PASSWORD=""
+SMTP_USERNAME="${CLOUDRON_MAIL_SMTP_USERNAME:-}"
+SMTP_PASSWORD="${CLOUDRON_MAIL_SMTP_PASSWORD:-}"
 SMTP_EMAIL="${CLOUDRON_MAIL_FROM:-no-reply@$RP_ID}"
 SMTP_SENDER_NAME="${CLOUDRON_MAIL_FROM_DISPLAY_NAME:-Ente}"