refactor: 移除备份恢复功能(API/CLI/手册)

日志数据的保留与安全由对象存储 + Loki retention 承担,平台侧整库备份恢复价值有限:
删除 services/backup.go、/system/{backup,restore,restore/apply,backups} 路由与 handler、
CLI backup/restore 子命令、前端 api 方法与类型;自检所需的 dataDirPath/密钥指纹并入 selfcheck;
用户手册删备份章节并修正 FAQ 升级建议;CHANGELOG 记录移除。

backend/services/selfcheck.go

@@ -1,10 +1,14 @@
 package services
 
 import (
+	"crypto/sha256"
+	"encoding/hex"
 	"encoding/json"
 	"fmt"
+	"lokistack/config"
 	"net/http"
 	"os"
+	"path/filepath"
 	"strings"
 	"syscall"
 	"time"
@@ -263,6 +267,11 @@ func fileDBStats(path string) string {
 		count("deployments"), count("collectors"), count("auditLogs"))
 }
 
+// dataDirPath 数据目录绝对路径(自检磁盘空间等场景使用)
+func dataDirPath() string {
+	return config.GetConfig().DataDir
+}
+
 // diskUsage 返回剩余与总空间（字节）
 func diskUsage(path string) (free, total uint64, err error) {
 	var st syscall.Statfs_t
@@ -271,3 +280,18 @@ func diskUsage(path string) (free, total uint64, err error) {
 	}
 	return st.Bavail * uint64(st.Bsize), st.Blocks * uint64(st.Bsize), nil
 }
+
+// encryptionKeyFingerprint ENCRYPTION_KEY 指纹(SHA256 前 16 位,自检展示用,不泄露密钥)
+func encryptionKeyFingerprint() string {
+	key := config.GetConfig().EncryptionKey
+	if key == "" {
+		if b, err := os.ReadFile(filepath.Join(config.GetConfig().DataDir, ".encryption-key")); err == nil {
+			key = strings.TrimSpace(string(b))
+		}
+	}
+	if key == "" {
+		return ""
+	}
+	sum := sha256.Sum256([]byte(key))
+	return hex.EncodeToString(sum[:])[:16]
+}