Playground Browser Upload

Browser-direct playground upload, token-authed (see _verify_playground_upload_token). The dashboard holds the customer session and mints the token; the browser POSTs the raw file bytes here.

This endpoint is on api.empiriolabs.ai, which is NOT behind the platform pull zone that silently caps POST bodies near 10 MB, so large videos and audio arrive intact (verified 2026-06-30: a 22 MB body round-trips here while the same upload through platform.empiriolabs.ai truncated to ~10 MB). Returns the stored path + a signed URL; the dashboard records the playground_uploads row via its own commit route (this service holds no Supabase credentials).