"""Email group domain — Helix-side mirror of Google Workspace Groups.

Google is the source of truth for membership; Helix keeps its own row
so offboarding ``_rescan()`` can find groups a leaver owns in one SQL
query instead of walking every Google group. A scheduled sync job
reconciles drift.

This domain owns only the ``email_groups`` + ``email_group_owners``
tables. The actual mutation (create / delete / add-alias / transfer
owner) is performed by the Google Workspace driver — this domain
*coordinates* the write-through: local row first, Google second,
rollback on failure.
"""