feat(V2): migrazione alpaca completa

Task 6.7: porting alpaca da services/mcp-alpaca a src/cerbero_mcp.
client.py + leverage_cap.py copiati 1:1 (default cap 1 cash).
tools.py: 17 tool senza ACL/Principal/audit. Router /mcp-alpaca con 18
route (env_info + 17 tool). Builder branch alpaca: paper=(env=="testnet"),
api_key viene da settings.alpaca.api_key_id. Test client + leverage_cap
migrati (15 test alpaca pass). Test builder con stub SDK alpaca-py.

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

tests/unit/exchanges/alpaca/test_leverage_cap.py

@@ -0,0 +1,46 @@
+from __future__ import annotations
+
+import pytest
+from fastapi import HTTPException
+from cerbero_mcp.exchanges.alpaca.leverage_cap import enforce_leverage, get_max_leverage
+
+
+def test_get_max_leverage_returns_creds_value():
+    creds = {"max_leverage": 4}
+    assert get_max_leverage(creds) == 4
+
+
+def test_get_max_leverage_default_when_missing():
+    """Default 1 (cash) se il secret non ha max_leverage."""
+    assert get_max_leverage({}) == 1
+
+
+def test_enforce_leverage_pass_at_cap_one():
+    """Alpaca cash account: cap 1, leverage 1 OK."""
+    creds = {"max_leverage": 1}
+    enforce_leverage(1, creds=creds, exchange="alpaca")  # no raise
+
+
+def test_enforce_leverage_reject_over_cap_one():
+    creds = {"max_leverage": 1}
+    with pytest.raises(HTTPException) as exc:
+        enforce_leverage(2, creds=creds, exchange="alpaca")
+    assert exc.value.status_code == 403
+    assert exc.value.detail["error"] == "LEVERAGE_CAP_EXCEEDED"
+    assert exc.value.detail["exchange"] == "alpaca"
+    assert exc.value.detail["requested"] == 2
+    assert exc.value.detail["max"] == 1
+
+
+def test_enforce_leverage_reject_when_below_one():
+    creds = {"max_leverage": 1}
+    with pytest.raises(HTTPException) as exc:
+        enforce_leverage(0, creds=creds, exchange="alpaca")
+    assert exc.value.status_code == 403
+
+
+def test_enforce_leverage_default_when_none():
+    """Se requested è None, applica il cap come default."""
+    creds = {"max_leverage": 1}
+    result = enforce_leverage(None, creds=creds, exchange="alpaca")
+    assert result == 1