feat(api): add system resource monitoring functionality

- implement getSystemInfo to gather CPU and memory usage
- add platform-specific implementations for memory and CPU usage
- enhance OpenAPI documentation to include system resource metrics

internal/api/system_windows.go

@@ -0,0 +1,77 @@
+//go:build windows
+
+package api
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+var (
+	kernel32              = syscall.NewLazyDLL("kernel32.dll")
+	globalMemoryStatusEx  = kernel32.NewProc("GlobalMemoryStatusEx")
+	getSystemTimes        = kernel32.NewProc("GetSystemTimes")
+)
+
+// MEMORYSTATUSEX structure
+type memoryStatusEx struct {
+	dwLength                uint32
+	dwMemoryLoad            uint32
+	ullTotalPhys            uint64
+	ullAvailPhys            uint64
+	ullTotalPageFile        uint64
+	ullAvailPageFile        uint64
+	ullTotalVirtual         uint64
+	ullAvailVirtual         uint64
+	ullAvailExtendedVirtual uint64
+}
+
+func getMemoryInfo() (total, used uint64) {
+	var ms memoryStatusEx
+	ms.dwLength = uint32(unsafe.Sizeof(ms))
+
+	ret, _, _ := globalMemoryStatusEx.Call(uintptr(unsafe.Pointer(&ms)))
+	if ret == 0 {
+		return 0, 0
+	}
+
+	return ms.ullTotalPhys, ms.ullTotalPhys - ms.ullAvailPhys
+}
+
+type filetime struct {
+	dwLowDateTime  uint32
+	dwHighDateTime uint32
+}
+
+func (ft filetime) ticks() uint64 {
+	return uint64(ft.dwHighDateTime)<<32 | uint64(ft.dwLowDateTime)
+}
+
+var prevIdleWin, prevTotalWin uint64
+
+func getCPUUsage() float64 {
+	var idleTime, kernelTime, userTime filetime
+
+	ret, _, _ := getSystemTimes.Call(
+		uintptr(unsafe.Pointer(&idleTime)),
+		uintptr(unsafe.Pointer(&kernelTime)),
+		uintptr(unsafe.Pointer(&userTime)),
+	)
+	if ret == 0 {
+		return 0
+	}
+
+	idle := idleTime.ticks()
+	total := kernelTime.ticks() + userTime.ticks() // kernel includes idle
+
+	deltaTotal := total - prevTotalWin
+	deltaIdle := idle - prevIdleWin
+	prevIdleWin = idle
+	prevTotalWin = total
+
+	if deltaTotal == 0 {
+		return 0
+	}
+
+	return float64(deltaTotal-deltaIdle) / float64(deltaTotal) * 100
+}