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