Admin Email Campaign Coverage

Eligible users who have no recipient row on this campaign.

The gap a campaign's own counters cannot show. Computed live against current eligibility, so it answers "who is still missing this?" rather than what was true at creation (that is the stored coverage_gap).

joined_after_campaign separates the benign case — people who registered after the campaign was built, who were never candidates — from users who existed and were simply not selected.