Support for service_tier="fast"

Right now, we can set the effort level in `~/.config/claude-code-proxy/config.json`, such as

```json
{
  "port": 18765,
  "codex": {
    "model": "gpt-5.5",
    "effort": "xhigh"
  },
  "log": {
    "stderr": true,
    "verbose": false
  }
}
```

It would be better if the setting of `"service_tier":"fast"` could be supported.