fix: 按 review 意见重构数据库备份服务（安全性 + 架构 + 健壮性）

1. S3 凭证加密存储：使用 SecretEncryptor (AES-256-GCM) 加密 SecretAccessKey，
   防止备份文件中泄露 S3 凭证，兼容旧的未加密数据
2. 修复 saveRecord 竞态条件：添加 recordsMu 互斥锁保护 records 的 load/save
3. 恢复操作增加服务端验证：handler 层要求重新输入管理员密码，通过 bcrypt
   校验，前端弹出密码输入框
4. pg_dump/psql/S3 操作抽象为接口：定义 DBDumper 和 BackupObjectStore 接口，
   实现放入 repository 层，遵循项目依赖注入架构规范
5. 改为流式处理避免大数据库 OOM：备份时 pg_dump stdout -> gzip -> io.Pipe ->
   S3 upload；恢复时 S3 download -> gzip reader -> psql stdin，不再全量加载
6. loadRecords 区分"无数据"和"数据损坏"场景：JSON 解析失败返回明确错误
7. 添加 18 个核心逻辑单元测试：覆盖加密、并发、流式备份/恢复、错误处理等

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/handler/admin/backup_handler.go

@@ -2,16 +2,21 @@ package admin
 
 import (
 	"github.com/Wei-Shaw/sub2api/internal/pkg/response"
+	"github.com/Wei-Shaw/sub2api/internal/server/middleware"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 	"github.com/gin-gonic/gin"
 )
 
 type BackupHandler struct {
 	backupService *service.BackupService
+	userService   *service.UserService
 }
 
-func NewBackupHandler(backupService *service.BackupService) *BackupHandler {
-	return &BackupHandler{backupService: backupService}
+func NewBackupHandler(backupService *service.BackupService, userService *service.UserService) *BackupHandler {
+	return &BackupHandler{
+		backupService: backupService,
+		userService:   userService,
+	}
 }
 
 // ─── S3 配置 ───
@@ -154,7 +159,11 @@ func (h *BackupHandler) GetDownloadURL(c *gin.Context) {
 	response.Success(c, gin.H{"url": url})
 }
 
-// ─── 恢复操作 ───
+// ─── 恢复操作（需要重新输入管理员密码） ───
+
+type RestoreBackupRequest struct {
+	Password string `json:"password" binding:"required"`
+}
 
 func (h *BackupHandler) RestoreBackup(c *gin.Context) {
 	backupID := c.Param("id")
@@ -162,6 +171,31 @@ func (h *BackupHandler) RestoreBackup(c *gin.Context) {
 		response.BadRequest(c, "backup ID is required")
 		return
 	}
+
+	var req RestoreBackupRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.BadRequest(c, "password is required for restore operation")
+		return
+	}
+
+	// 从上下文获取当前管理员用户 ID
+	sub, ok := middleware.GetAuthSubjectFromContext(c)
+	if !ok {
+		response.Unauthorized(c, "unauthorized")
+		return
+	}
+
+	// 获取管理员用户并验证密码
+	user, err := h.userService.GetByID(c.Request.Context(), sub.UserID)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+	if !user.CheckPassword(req.Password) {
+		response.BadRequest(c, "incorrect admin password")
+		return
+	}
+
 	if err := h.backupService.RestoreBackup(c.Request.Context(), backupID); err != nil {
 		response.ErrorFrom(c, err)
 		return