Let's provide users' `last_used`.

### What Operating System(s) are you seeing this problem on?

Linux (x86-64)

### Backend.AI version

996b053

### Describe the bug

[Keypair's last_used](https://github.com/lablup/backend.ai/blob/main/src/ai/backend/manager/models/keypair.py#L79) doesn't seem to work because I got always a null value from that.

### To Reproduce

query `last_used` from the keypair table.

### Expected Behavior

return last used date.

### Anything else?

_No response_

JIRA Issue: BA-209