[feature] Add option to disable cross organization registration

Is your feature request related to a problem? Please describe.
Currently, if a user is registered with Org A, they are able to log in to Org B without needing to register again, as long as registration is enabled for Org B. This behavior was introduced in commit 0472212, but there is currently no way to disable this functionality. It can be required to prevent cross organization registration for security or policy reasons.

Describe the solution you'd like
Introduce a setting that allows administrators to disable or enable cross organization registration. This setting should make it possible to require users to register separately for each organization, even if they have already registered with another organization.

Additional context
Related feature was added in commit 0472212d13171aa10d3b55336f6aaec72bd73201.
This configuration option would improve security and give deployments more control over user management across organizations.

Start by inspecting commit 0472212 and the registration and login flow it introduced. Locate the existing organization-related configuration and determine where a new setting would control cross-organization registration. Done means administrators can enable or disable the behavior and users must register separately when it is disabled, with tests covering both settings.