* fix: make a campaign's linked segments the audience rather than an accumulator, so detaching one withdraws the leads it enrolled instead of leaving the old list mixed in with the new, tracked by a new campaign_leads.source that keeps a hand-picked lead, an overlapping segment's member and anyone the campaign has already emailed out of the withdrawal, and reported back as withdrawn/contacted counts the dialog confirms and explains (issue #510)
* fix: serialize the linked-segment sweep against a link replacement by taking the same campaign lock, so a sweep that read the old set cannot re-enrol the audience the replacement just withdrew, and word the dialog's confirm and toast so the campaign, not the segment, is what has already emailed a lead
* fix: stop the one-shot segment enrol from re-stamping leads that are already in the campaign, since the Leads tab's Add back runs through it and pinning a whole linked audience as hand-picked because one held-out member was restored is the accumulation this change exists to end
* fix: lock the leads a detachment is about to withdraw before deciding, because a send is reserved by stamping campaign_contact_progress and only then locking the lead row, so a reservation committing mid-pass was invisible to the delete's snapshot and could withdraw a lead whose first email had already gone; and report the already-emailed count on every toast branch, since a detach where the whole audience had been emailed changed no count and said nothing after confirming a removal
* fix: add the campaign_leads.source check constraint NOT VALID, which still enforces every insert and update while skipping a full scan of the largest table in the product under ACCESS EXCLUSIVE to learn that a one-statement-old column holds its own default everywhere
Matthew Meszaros