Per-outcome price history (v2)

Per-outcome Kalshi/Polymarket ask history for one pair, oldest->newest, for the /live/[id] chart.

Accepts dino_<uuid>, a bare UUID, or a persisted poly_slug. Empty series when the sampler hasn't captured this pair yet (a brand-new pair, or one seen only within the current sample bucket) — never a 404 for that.

§131b: trims trailing points newer than now - REST_DELAY_S (same uniform delay as every other REST price surface — a chart must not show the price ~2min ahead of what /v2/markets/pairs serve for the same pair). The sampler buckets at _PH_BUCKET_S (60s), so this drops at most ceil(delay/60)+1 points (~2 points at the 120s default).