feat(V2): IBKR client base + auth header + tickle keep-alive

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

src/cerbero_mcp/exchanges/ibkr/leverage_cap.py

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+"""Leverage cap server-side per place_order (IBKR Reg-T context).
+
+Cap letto dal secret JSON via campo `max_leverage`. Default 1 (cash) se assente.
+IBKR margin accounts default a 4x intraday / 2x overnight (Reg-T).
+"""
+from __future__ import annotations
+
+from fastapi import HTTPException
+
+
+def get_max_leverage(creds: dict) -> int:
+    """Legge max_leverage dal secret. Default 1 se mancante."""
+    raw = creds.get("max_leverage", 1)
+    try:
+        value = int(raw)
+    except (TypeError, ValueError):
+        value = 1
+    return max(1, value)
+
+
+def enforce_leverage(
+    requested: int | float | None,
+    *,
+    creds: dict,
+    exchange: str,
+) -> int:
+    """Verifica e applica leverage cap. Ritorna leverage applicabile.
+
+    Solleva HTTPException(403, LEVERAGE_CAP_EXCEEDED) se requested > cap.
+    Se requested is None, applica il cap come default.
+    """
+    cap = get_max_leverage(creds)
+    if requested is None:
+        return cap
+    lev = int(requested)
+    if lev < 1:
+        raise HTTPException(
+            status_code=403,
+            detail={
+                "error": "LEVERAGE_CAP_EXCEEDED",
+                "exchange": exchange,
+                "requested": lev,
+                "max": cap,
+                "reason": "leverage must be >= 1",
+            },
+        )
+    if lev > cap:
+        raise HTTPException(
+            status_code=403,
+            detail={
+                "error": "LEVERAGE_CAP_EXCEEDED",
+                "exchange": exchange,
+                "requested": lev,
+                "max": cap,
+            },
+        )
+    return lev