Cancel an in-flight society creation job from the iframe.

Same auth model as the poll endpoint: HMAC over job_id with the server's signing secret. The Cancel button in the rich-UI panel POSTs here; we proxy the call to digital-twin-api's POST /pipelines/jobs/{job_id}/cancel using the service token, then return the freshest society_progress payload so the iframe re- renders to the cancelled state without waiting for the next poll tick.

DT-API's cancel is COOPERATIVE on running jobs (worker sees the flag at the next step boundary, unwinds, runs artifact cleanup) and a direct status flip on queued jobs. Both paths return JSON with the new status; we don't try to distinguish them here.

Idempotent — cancelling an already-terminal job returns its current state.