Fix silent failures and false success pings

- Collapse folded Message-ID headers before IMAP SEARCH; embedded
  CRLF was breaking the command and crashing every run
- Set Type=oneshot so the success ping only fires on clean exit
  instead of on every spawn, letting OnFailure report real failures

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

email_forwarder.service

@@ -3,6 +3,7 @@ Description=Email Forwarder
 OnFailure=email_forwarder-fail-notify.service
 
 [Service]
+Type=oneshot
 WorkingDirectory=${PROJECT_DIR}
 ExecStart=${PROJECT_DIR}/venv/bin/python -m email_forwarder
 EnvironmentFile=${PROJECT_DIR}/.env