Self-hosted: allow one sending domain across multiple workspaces

Scope & Context We run one self-hosted instance with a workspace per client business. Our team works across these workspaces and sends campaigns as [email protected]. A sending domain can only be registered in one workspace per instance, so only one workspace can send as ourcompany.com. This makes sense as the default on Twenty Cloud, but on a self-hosted instance one organization owns every workspace. Current behavior • Adding an email channel on a domain already used in another workspace fails with "Registered by another workspace." • EmailingDomainService.createEmailingDomain checks domain against all workspaces and throws EMAILING_DOMAIN_ALREADY_REGISTERED. • Removing the last email channel on a domain deletes the domain from the provider account (cleanupDomain). Expected behavior A verified domain can be used by more than one workspace on a self-hosted instance. Any of these would work: 1. A server setting such as EMAILING_DOMAIN_ALLOW_MULT…