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.go

@@ -0,0 +1,16 @@
+package api
+
+type systemInfo struct {
+	CPUUsage float64 `json:"cpu_usage"` // percent 0-100
+	MemTotal uint64  `json:"mem_total"` // bytes
+	MemUsed  uint64  `json:"mem_used"`  // bytes
+}
+
+func getSystemInfo() systemInfo {
+	memTotal, memUsed := getMemoryInfo()
+	return systemInfo{
+		CPUUsage: getCPUUsage(),
+		MemTotal: memTotal,
+		MemUsed:  memUsed,
+	}
+}