Historical candle store + /v1/candles endpoint

[ozpool]

Problem

PriceChart paints 240 synthetic candles via seedCandles(anchor, 240) on mount. Only the right-most candle is live (oracle ticks update its OHLC). Anything left of "now" is decorative — refresh the page and the entire history changes.

Proposal

Persist OHLC buckets inside the edge per market per timeframe.

• New BucketStore in AppState: RwLock<HashMap<(MarketId, Tf), VecDeque<Candle>>>.
• On every record_fill, find the active bucket for each supported Tf (1m, 5m, 15m, 1h, 4h, 1d) and update high/low/close + volume.
• Roll the bucket forward when wall clock crosses the next boundary.
• New REST endpoint:

  GET /v1/candles/:marketId?tf=1m&limit=240
  

  Returns Candle[] with { time, open, high, low, close, volume }.
• FE: PriceChart.useEffect swaps seedCandles(...) for an initial fetch of /v1/candles.

Acceptance

• Refreshing /trade keeps the previous history — only the live tail extends.
• cargo test green; new test fills two orders one minute apart and asserts two distinct candles.
• FE: chart history persists across page refresh.

Notes

Unblocks #3 (real chart volume bars) and #4 (real 24h change %).