Get per-event price history

Returns one row per onchain event that touched the outcome (mint/redeem/etc), from the outcome_stats hypertable. Unlike /prices/history this is NOT bucketed or gap-filled — each point reflects the marginal price and collateral immediately after a specific event, so the chart can plot per-trade resolution for short windows. Use start_ts/end_ts to bound the window (required for start_ts since the hypertable can be large); use fidelity to downsample if the window contains more events than you want to render. Events before the outcome's first real trade are omitted — the market "seed" mint carries a distorted bootstrap price (not a real traded price), and that price is carried forward until the outcome is first traded, so the series opens on a genuine price.