fix(state): repository iv_rv_history time-stable + input validation

Risponde alla code review di 395191e:
- iv_rv_history accetta as_of (default now UTC) invece di
  affidarsi al clock SQLite, rendendo i test time-stable.
- Valida max_days > 0 e raise se as_of/reference sono naive.
- Aggiunge 3 test sulle nuove guard.

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

src/cerbero_bite/state/repository.py

@@ -414,20 +414,31 @@ class Repository:
         *,
         asset: str,
         max_days: int,
+        as_of: datetime | None = None,
     ) -> list[Decimal]:
-        """Lista IV-RV ordinata ASC sull'intervallo `[now-max_days, now]`.
+        """Lista IV-RV ordinata ASC sull'intervallo `[as_of - max_days, as_of]`.
 
         Esclude righe con ``fetch_ok=0`` o ``iv_minus_rv IS NULL``.
         Usata dal validator quando il gate adattivo è abilitato.
+
+        Args:
+            as_of: Reference time for the rolling window. Defaults to
+                ``datetime.now(UTC)``. Tests can pin a fixed value.
         """
+        if max_days <= 0:
+            raise ValueError(f"max_days must be positive, got {max_days}")
+        ref = as_of if as_of is not None else datetime.now(UTC)
+        if ref.tzinfo is None:
+            raise ValueError("as_of must be timezone-aware")
+        cutoff = ref - timedelta(days=max_days)
         rows = conn.execute(
             "SELECT iv_minus_rv FROM market_snapshots "
             "WHERE asset = ? "
             "  AND fetch_ok = 1 "
             "  AND iv_minus_rv IS NOT NULL "
-            "  AND timestamp >= datetime('now', ?) "
+            "  AND timestamp >= ? "
             "ORDER BY timestamp ASC",
-            (asset, f"-{int(max_days)} days"),
+            (asset, _enc_dt(cutoff)),
         ).fetchall()
         return [Decimal(str(r["iv_minus_rv"])) for r in rows]
 
@@ -445,6 +456,8 @@ class Repository:
         ``dvol IS NOT NULL``) entro la tolerance. Usato dal Vol-of-Vol
         guard per stimare DVOL N ore fa.
         """
+        if reference.tzinfo is None:
+            raise ValueError("reference must be timezone-aware")
         ref_lo = (reference - timedelta(minutes=tolerance_minutes)).astimezone(UTC)
         ref_hi = (reference + timedelta(minutes=tolerance_minutes)).astimezone(UTC)
         row = conn.execute(