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>

src/cerbero_mcp/exchanges/__init__.py

@@ -36,4 +36,12 @@ async def build_client(
             testnet=(env == "testnet"),
             api_wallet_address=settings.hyperliquid.api_wallet_address,
         )
+    if exchange == "alpaca":
+        from cerbero_mcp.exchanges.alpaca.client import AlpacaClient
+
+        return AlpacaClient(
+            api_key=settings.alpaca.api_key_id,
+            secret_key=settings.alpaca.secret_key.get_secret_value(),
+            paper=(env == "testnet"),
+        )
     raise ValueError(f"unsupported exchange: {exchange}")