feat(V2): /health/ready con ping client + middleware request log strutturato + request_id correlation

- /health/ready: ping di tutti i client (exchange, env) cached con
  timeout 2s, status ready|degraded|not_ready, opt-in 503 via
  READY_FAILS_ON_DEGRADED.
- Middleware mcp.request: 1 riga JSON per HTTP request con request_id,
  method, path, status_code, duration_ms, actor, bot_tag, exchange,
  tool, client_ip, user_agent.
- request_id propagato in request.state, audit log e error envelope per
  correlazione cross-cutting.
- Aggiunto async health() come probe minimo a bybit/alpaca/macro/
  sentiment/deribit (hyperliquid lo aveva già).

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

src/cerbero_mcp/exchanges/alpaca/client.py

@@ -99,6 +99,10 @@ class AlpacaClient:
         if not self._http.is_closed:
             await self._http.aclose()
 
+    async def health(self) -> dict[str, Any]:
+        """Probe minimo per /health/ready: nessuna chiamata di rete."""
+        return {"status": "ok", "paper": self.paper}
+
     # ── Helpers ──────────────────────────────────────────────────
 
     @property