Move REDIS to engine dir (#16006)

### What problem does this PR solve?

as title.

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

internal/admin/handler.go

@@ -21,10 +21,10 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
-	"ragflow/internal/cache"
 	"ragflow/internal/common"
 	"ragflow/internal/dao"
 	"ragflow/internal/engine"
+	"ragflow/internal/engine/redis"
 	"ragflow/internal/server"
 	"ragflow/internal/service"
 	"ragflow/internal/utility"
@@ -151,7 +151,7 @@ func (h *Handler) Login(c *gin.Context) {
 		return
 	}
 
-	secretKey, err := server.GetSecretKey(cache.Get())
+	secretKey, err := server.GetSecretKey(redis.Get())
 	if err != nil {
 		c.JSON(http.StatusOK, gin.H{
 			"code":    common.CodeServerError,