Support multiple companies (branches, subsidiaries) sharing one domain: parent company relation + group-aware domain matching

Scope & Context Many real-world companies consist of several legal entities or locations that share one website and one email domain: retail chains with branches, franchises, subsidiaries of a group, regional offices. Typical B2B example: a hardware chain whose 100+ branches all use chain.com, each with its own address, contact persons, deals and ERP customer number. Twenty treats Company.domainName (primary link) as unique. This is a sound assumption for contact/domain matching, but it leaves no documented way to model "many companies, one domain". Everybody who imports an existing customer base with branches runs into this, and today each team finds its own workaround. Current behavior 1. A unique index exists on company.domainNamePrimaryLinkUrl. Creating or importing a second live company with the same domain fails with a 400 (duplicate key value violates unique constraint "IDX_UNIQUE_…"). Since 2.40 soft-deleted companies no longer block the value (#262…