fix: Phase 6.2 - Health 端点对齐接口文档 (uptime_seconds 整数格式)

backend/cmd/server/main.go

@@ -106,7 +106,7 @@ func healthHandler(sessionMgr session.Manager) gin.HandlerFunc {
 		c.JSON(200, gin.H{
 			"status":          "ok",
 			"version":         "0.1.0",
-			"uptime":          time.Since(startTime).String(),
+			"uptime_seconds":  int(time.Since(startTime).Seconds()),
 			"active_sessions": sessionMgr.ActiveCount(),
 		})
 	}