[TobiasMende]

Adds a Mail Import: a one-off import of past mail from chosen folders of an IMAP Email Account into Communications.

• Scan reads headers of the chosen folders (subfolders included, optional date range) in resumable background jobs, read-only (BODY.PEEK, read-only selects).
• Review lists addresses that are no Contact yet: create a Contact (optionally linked to a company), add the address to an existing Contact, or ignore it site-wide.
• Import imports only mails with at least one Contact, as Closed and seen Communications (Sent when the sender is an own address). Deduplicated by Message-ID (or account, folder and UID), never creates contacts on its own, 25 MB per mail.
• Import-only mailboxes are normal Email Accounts with Enable Incoming off, so they are never polled.
• Only System Managers and users with the account in their User Emails can run imports.
• New hooks for apps: mailsync_own_address, mailsync_address_suggestion, mailsync_contact_link_doctype. mailsync_recipient_links and mailsync_mail_read also run for imported mails. frappe.flags.mailsync_import is set while an imported Communication is inserted.

Design: docs/superpowers/specs/2026-10-28-imap-backfill-design.md, plan: docs/superpowers/plans/2026-10-28-imap-backfill.md.

Tests cover scan, review, import, jobs and access, plus an end-to-end run against GreenMail that checks no mail gets \Seen.

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