This PR limits IMAP email fetching to 500 messages per sync run to avoid
expensive/long-running mailbox scans. It also filters out
already-imported emails and Chatwoot-generated notification emails
during the header fetch phase, before fetching full email bodies,
reducing unnecessary IMAP work.
Fixes #CW-7001 (issue) :
https://linear.app/chatwoot/issue/CW-7001/emails-not-syncing