failed to get last fit ranking model time

**Gorse version**
```
Version: v0.4.15
API version: v0.2.7
Go version: go1.20.12
Git commit: 09ad3aa8cdf4c46408f774668ff2fffa458a34ed
Built: Wed Jan 10 04:45:06 UTC 2024
OS/Arch: linux/amd64
```

**Describe the bug**
I see in logs

`{"level":"warn","ts":1721912611.414353,"msg":"failed to get last fit ranking model time","request_id":"fadb4ad1-de67-4ef3-b7c3-2350df763d8b","error":"global_meta/last_fit_ranking_model_time: object not found","errorVerbose":"github.com/zhenghaoz/gorse/storage/cache.init:132: object not found\ngithub.com/zhenghaoz/gorse/storage/cache.(*Redis).Get:299: global_meta/last_fit_ranking_model_time"}`

**To Reproduce**
- use redis for cache
- use postgres for database
- prefix set to gorse_

the rest is same in config