OHLC candles for one coin

HL `candleSnapshot` window. `interval` ∈ { 1m, 5m, 15m, 1h, 4h, 1d }. `limit` clamped to [1, 500]; default 100. Cached 30s (the most recent candle is still forming — clients refreshing more often than that get the same in-flight value). Three windowing strategies — pick whichever fits the caller: 1. Neither `start_time` nor `end_time` → most recent `limit` candles (default). 2. Both `start_time` + `end_time` → all buckets in `[start_time, end_time)`, capped at `limit` newest. 3. Only `start_time` → from that timestamp forward, capped at `limit`. 4. Only `end_time` → ending at that timestamp, the most recent `limit` candles. `start_time` and `end_time` accept either Unix-ms (number) or an ISO-8601 string (e.g. `2026-05-19T00:00:00Z`).