A display's DHCP IP can change; its FQDN (F<serial>.<domain>, domain from the
display_fqdn_domain setting) is stable and the collector already reports the
serial. Add a nullable unique fqdn column (varchar191 so the index fits utf8mb4
without innodb_large_prefix), make ipaddress nullable, and require fqdn OR ip.
visitor-location + display-role resolve by FQDN first, then IP; create/update
accept fqdn. Core migration 7d31, verified up/down/idempotent on MySQL 5.6.
'Business unit' wording -> 'location' in the validation messages.
