OHLC candles for one spot pair

HL `candleSnapshot` window. `interval` ∈ { 1m, 5m, 15m, 1h, 4h, 1d }. `limit` clamped to [1, 500]; default 100. Cached 30s. Four windowing strategies: 1. Neither `start_time` nor `end_time` → most recent `limit` candles. 2. Both `start_time` + `end_time` → 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`. `start_time` and `end_time` accept either Unix-ms or an ISO-8601 string.