Cross-provider price compare for one universal event

Resolves the `universalId` via the event registry, then fetches the current quote from every linked provider market in parallel and normalizes the result to `(yesPrice, noPrice)` in 0..1 probability units. Per-leg upstream failures surface as `yesPrice: null` + a `warning` string inside `raw`, never as a 5xx for the whole response. Cached 5s per `universalId`.