Runs against a database where every user was created by a gaming sign-in, so
most rows have no email at all and nothing has ever stopped two rows from
sharing one. The address is normalized first, duplicates are separated before
the unique index exists — the older row keeps the address, the newer one is
asked for a new one and loses nothing else — and the index is partial so that
accounts with no address do not collide with each other.
Verified against a database built to contain the awkward rows rather than
against an empty schema, by the script alongside it: an account with no
address, one with both, one with two connections, a duplicated address in two
different cases, an account already over the connection cap, and a deleted row
holding an address a live row also holds. Removing the de-duplication makes
the index creation fail, which is how we know the fixtures are load-bearing.
Also adds a nullable column recording which attempt holds a session run. It is
not part of the change above and carries no reason of its own; the endpoint
that reads and writes it arrives separately, and it is here because a schema
change has one owner at a time.
