Mail import: support@ runs hang 120s on UID FETCH, then 'Connection not available'

Imported from GitHub issue El-SaMa/oma#238 by @El-SaMa.

Seen 2026-10-15 on [email protected] (source 3fdfa5a9…): runs at 09:53 and 10:14 UTC logged uid 348/349: Connection not available. Glama dovecot shows Disconnected: Connection closed (UID FETCH finished 120.0 secs ago) for those sessions. The next run picks the mail up, so nothing is lost, but each hang delays new tickets by minutes. oma.admin_mail_import import_now for support@ also timed out on the MCP side while the import still finished.

Look at: mail-import-runner fetch loop (a message body fetch that never resolves until the server idle timeout), add a per-message fetch timeout and reconnect, and make import_now return before the MCP timeout (queue and report, or a shorter per-source budget). Add a test for the timeout path. Handbook line in support.md (mail import section).