feat(V2): shared Candle validator + uniform 'candles' response key

Introduce common/candles.py with a Pydantic Candle model enforcing OHLC
consistency (high≥max, low≤min), non-negative volume and positive
timestamp. validate_candles() coerces upstream rows, sorts by timestamp
and raises HTTPException(502) on malformed data — surfacing upstream
data corruption as a retryable envelope instead of silently returning
nonsense.

Wired into all five exchange historical endpoints (Bybit, Hyperliquid,
Deribit, Alpaca, IBKR). BREAKING: Alpaca get_bars and IBKR get_bars now
return 'candles' (was 'bars') to align with the other exchanges.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/exchanges/alpaca/test_client.py

@@ -271,8 +271,8 @@ async def test_get_bars_stocks(httpx_mock: HTTPXMock, client: AlpacaClient):
     )
     assert result["symbol"] == "AAPL"
     assert result["interval"] == "1d"
-    assert len(result["bars"]) == 1
-    assert result["bars"][0]["close"] == 175.0
+    assert len(result["candles"]) == 1
+    assert result["candles"][0]["close"] == 175.0
 
 
 @pytest.mark.asyncio