Kick a brand extraction from a URL

Start an async read of a site's brand (colours + font + a preview screenshot) via dash-render's /v1/extract-brand — its only outbound-network route, where the SSRF guard lives (B4). Returns a job_id; poll /jobs/{id} for the resolved {signals, screenshot} (rejected → {error}). Async, not the former in-request sync call: the extraction is a headless-Chrome navigation, which cannot run inside the browserless apiv2 pod and would hold a web worker for up to its 30s op timeout. The URL hygiene + SSRF guard ride along in the render task unchanged; the request body is unchanged so the FE only swaps its await for the same submit+poll pdf/video use.