Verify a sequence's emails

Check whether the addresses in a sequence's audience are deliverable, and have the sequence email only the ones whose verdict it accepts (valid alone unless the Audience tab or set_sequence_audience chose more). Spends credits: price it first with estimate_email_verification, over exactly the same addresses. Adds a Verify email column over the sequence's email column when the table has none, points the sequence's email deliverability at it, and runs it only on addresses it has not checked. Answers with the run: verdicts fill in row by row over the next minutes, and a contact's email step waits for theirs.