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>

src/cerbero_mcp/exchanges/alpaca/client.py

@@ -20,6 +20,7 @@ from typing import Any
 
 import httpx
 
+from cerbero_mcp.common.candles import validate_candles
 from cerbero_mcp.common.http import async_client
 
 # ── Endpoint base ────────────────────────────────────────────────
@@ -301,9 +302,17 @@ class AlpacaClient:
 
         bars_dict = (data or {}).get("bars") or {}
         rows = bars_dict.get(symbol) or []
-        bars = [
+
+        def _iso_to_ms(ts: str | int | None) -> int | None:
+            if ts is None or isinstance(ts, int):
+                return ts
+            return int(_dt.datetime.fromisoformat(
+                ts.replace("Z", "+00:00")
+            ).timestamp() * 1000)
+
+        candles = validate_candles([
             {
-                "timestamp": b.get("t"),
+                "timestamp": _iso_to_ms(b.get("t")),
                 "open": b.get("o"),
                 "high": b.get("h"),
                 "low": b.get("l"),
@@ -311,12 +320,12 @@ class AlpacaClient:
                 "volume": b.get("v"),
             }
             for b in rows
-        ]
+        ])
         return {
             "symbol": symbol,
             "asset_class": ac,
             "interval": interval,
-            "bars": bars,
+            "candles": candles,
         }
 
     async def get_snapshot(self, symbol: str) -> dict: