Offboarding v4.7.0

Fixes a crash that stopped an email migration partway through, discovered during a real offboarding.

The problem: if the mailbox you're migrating contains a malicious email, your endpoint antivirus can quarantine the message file on local disk right after GYB backs it up. The file still exists but can't be read, and GYB's restore dies when it reaches it. When this bit us, only 634 of 3070 messages had reached the successor, and the script carried on with suspension and licence removal as if nothing was wrong.

What's new in 4.7.0:

• Before restoring, the script checks that it can read every backed-up .eml file. It moves any unreadable file to a <backup>_quarantined/ folder next to the backup, so the restore skips it and finishes in one pass. Nothing gets deleted.
• The script writes each skipped message to <backup>_skipped-messages.csv (Gmail message ID, date, where the file went) and calls them out in red during the run and again in the final summary. To see what a skipped message actually was, search its ID in your antivirus quarantine log, Google Vault, or the Security Investigation Tool. Skipped messages stay out of the destination mailbox on purpose: they are the flagged mail.
• Fixed a misleading summary. A failed restore used to log "Email migrated" anyway; it now reports the failure and the retained backup path so you can resume.
• The startup update check now points at this releases page instead of assuming you cloned the repo.

Upgrade: download offboard_user.py from this release, or git pull if you cloned. No flag or config changes needed.