feat: Agent采集uptime, 后端持久化kernel/uptime/diskIO, 前端磁盘BPS+IOPS展示

- Agent: system.go 添加 readUptime(), SystemStat 携带 Uptime
- Backend: Server 模型新增 Kernel 字段, ReceiveReport 持久化 kernel+uptime
- Backend: /agent/latest 返回 kernel/uptime/port_info/disk_io_info/disk_total
- Frontend: 磁盘监控改为 BPS+IOPS (4张卡片), 系统信息修复内核/运行时长/监听端口/磁盘容量
- 图表单位从 MB/s 改为 KB/s, 添加 formatBPS 工具函数

backend/pkg/crypto/secretbox.go

@@ -0,0 +1,66 @@
+package crypto
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/base64"
+	"errors"
+	"io"
+)
+
+// SecretBox AES-256-GCM 加密器，用 sha256(key) 派生 32 字节密钥
+type SecretBox struct {
+	key []byte
+}
+
+// NewSecretBox 创建加密器
+func NewSecretBox(masterKey string) *SecretBox {
+	h := sha256.Sum256([]byte(masterKey))
+	return &SecretBox{key: h[:]}
+}
+
+// Encrypt 加密明文，返回 base64 编码的密文（含 12 字节 nonce 前缀）
+func (s *SecretBox) Encrypt(plaintext string) (string, error) {
+	block, err := aes.NewCipher(s.key)
+	if err != nil {
+		return "", err
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", err
+	}
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return "", err
+	}
+	ciphertext := gcm.Seal(nonce, nonce, []byte(plaintext), nil)
+	return base64.StdEncoding.EncodeToString(ciphertext), nil
+}
+
+// Decrypt 解密密文，plaintext 应为 Encrypt 的返回值
+func (s *SecretBox) Decrypt(cipherB64 string) (string, error) {
+	ciphertext, err := base64.StdEncoding.DecodeString(cipherB64)
+	if err != nil {
+		return "", err
+	}
+	block, err := aes.NewCipher(s.key)
+	if err != nil {
+		return "", err
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", err
+	}
+	nonceSize := gcm.NonceSize()
+	if len(ciphertext) < nonceSize {
+		return "", errors.New("ciphertext too short")
+	}
+	nonce, ciphertext := ciphertext[:nonceSize], ciphertext[nonceSize:]
+	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
+	if err != nil {
+		return "", err
+	}
+	return string(plaintext), nil
+}