fix(deribit): DVOL midnight — finestra 1D estesa a ieri+oggi

Alle 00:00 UTC Deribit non ha ancora costruito il candle 1D di oggi:
con start_date=oggi la response è vuota e il client tirava
McpDataAnomalyError ('neither latest nor candles'). Includendo ieri
nello start_date, candles[-1] resta valido come fallback.

Verificato sui dati raccolti: 3 fail consecutivi 2026-05-02/03/04 a
00:00 UTC su ETH, zero fail dal 2026-05-05 in poi (container
rebuildato in mezzo al periodo).

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

src/cerbero_bite/clients/deribit.py

@@ -9,7 +9,7 @@ the ``core/`` algorithms stay in their preferred numeric domain.
 from __future__ import annotations
 
 import re
-from datetime import UTC, datetime
+from datetime import UTC, datetime, timedelta
 from decimal import Decimal
 from typing import Any, Literal
 
@@ -167,9 +167,12 @@ class DeribitClient:
     ) -> Decimal:
         """Return the latest DVOL value for ``currency``."""
         when = (now or datetime.now(UTC)).astimezone(UTC)
+        # Window starts one day back so a tick fired exactly at 00:00 UTC
+        # — before Deribit has built today's 1D candle — still has
+        # yesterday's close to fall back on (see candles[-1] branch).
         body = {
             "currency": currency,
-            "start_date": (when.date()).isoformat(),
+            "start_date": (when.date() - timedelta(days=1)).isoformat(),
             "end_date": when.date().isoformat(),
             "resolution": "1D",
         }