Non-release builds fail on `latest`/`head` tags

### Problem

On the `latest`, `head`, `latest-alpine`, and `head-alpine` versions, only the `release` build works. Selecting any other build fails with:

> version "latest" is not a fully qualified release version

If this is a limitation, it would be nice to disable the unavailable build options in the UI instead of erroring. But I hope it's not as using debug build on latest version is useful.

### Repro
```sh
curl -s -X POST https://fiddle.clickhouse.com/api/images/prepare -d '{"version":"latest","build_type":"debug"}'
```
```
{"error":{"message":"version \"latest\" is not a fully qualified release version","code":400}}
```

Start at the /api/images/prepare API entry point and reproduce the request with version "latest" and build_type "debug". Trace how latest, head, latest-alpine, and head-alpine are validated against build types, then check the related UI handling of unavailable options. Done means supported non-release builds work for these tags, or the UI clearly disables options that remain unsupported.

Written by the indexing model from the issue text.