feat: 解决 CDC 去重、集中备份与首次初始化问题 (#105)

实现 CDC 内容寻址仓库、远程 Agent 中央中转备份与首次初始化体验，并补充安全校验、测试及双语文档。

Closes #94
Closes #101
Closes #104

server/internal/http/router.go

@@ -322,7 +322,9 @@ func NewRouter(deps RouterDependencies) *gin.Engine {
 			agent.POST("/commands/:id/result", agentHandler.SubmitCommandResult)
 			agent.GET("/tasks/:id", agentHandler.GetTaskSpec)
 			agent.POST("/records/:id", agentHandler.UpdateRecord)
+			agent.PUT("/records/:id/artifacts/:targetId", agentHandler.UploadArtifact)
 			agent.GET("/restores/:id/spec", agentHandler.GetRestoreSpec)
+			agent.GET("/restores/:id/artifact", agentHandler.DownloadRestoreArtifact)
 			agent.POST("/restores/:id", agentHandler.UpdateRestore)
 
 			// Agent v1（安装脚本探活用），仅 Self 端点