Invites the same email (with one role) to multiple companies at once, sharing
a single invitation token and sending a single email.

The invitation is partial: companies where the email cannot be invited are
skipped and the rest are processed. Each skipped company comes back in
skipped with a stable reason:

• already_member: the email already belongs to a member of that company.
• owner_invitation: the email has a pending owner invitation there and the
  caller is not an owner of that company, so it cannot be moved.
• role_not_found: the company does not define the requested role.
• already_invited: another invitation for that company was created at the
  same time.

A pending invitation for the same email in one of the selected companies is
not skipped: it is moved to the new token with the new role and returned in
reinvited. Pending invitations in companies that were not selected keep
their previous link.

Permissions are not skipped: the caller needs members_management in every
target company, and assigning the owner role requires the caller to be an
owner of each one; otherwise the whole request is rejected with 403.

At most 200 companies per request: larger lists are rejected with 422
(too_many_companies) and nothing is processed, so split them into batches of
up to 200.

Every 403, 412 and 422 carries a stable code (forbidden,
owner_only, parameter_missing, too_many_companies, invalid_companies,
nothing_to_invite); branch on it instead of on the English message.