Continue Session

Create a continuation session under the SAME interview as the parent.

No auth required — session_id acts as the authorization, same as join endpoints. Delegates to _join_for_interview after resolving session → interview, so pause-resume goes through the same find-or-create code path as a fresh /join click. Terminal states return 400 to preserve the prior contract; interview-keyed callers can pass allow_re_completion=True (template setting) to opt into Phase 3 re-open behavior instead.