fix(doctor): close status-only memory manager after probe

The doctor.memory.status handler creates a memory manager with
purpose: "status" but never closes it. For qmd backends this opens
a read-only SQLite handle that only releases on close(), so repeated
health probes can accumulate file descriptors in long-lived gateway
processes. Add a finally block to ensure the manager is always closed
after the probe completes.

src/gateway/server-methods/doctor.ts

@@ -55,6 +55,8 @@ export const doctorHandlers: GatewayRequestHandlers = {
         },
       };
       respond(true, payload, undefined);
+    } finally {
+      await manager.close();
     }
   },
 };