Public Template Join

Spawn a fresh interview+session against a template, with no auth.

This is the entry point for generic share links (/join/t/<templateId>). Each call creates a new anonymous person row, a new interview, and a new session — there is no find-or-create, because we don't know who's clicking and we don't want two strangers walking into each other's conversation.

Security posture: the template_id is the bearer token (opaque UUID), same as how /v1/interviews/{id}/join treats the interview_id. The template must exist and have status='active' to be joinable. Org owners revoke public sharing by deactivating the template.