athens hanging with go mod download

**Describe the bug**

Using the go.mod provided, do `go clean -modcache` and then do a `go mod download`. The download will hang.

**Error Message**
If applicable add error message to help explain your problem or console output enclose in triple back-ticks eg,

```console

> go mod download

go: downloading go1.25.0 (linux/amd64)

go: download go1.25.0: golang.org/toolchain@v0.0.1-go1.25.0.linux-amd64: reading https://depot.wamblee.org/repository/wamblee-go-proxy/golang.org/toolchain/@v/v0.0.1-go1.25.0.linux-amd64.zip: 504 Gateway Timeout


```

**To Reproduce**
Steps to reproduce the behavior:

* `go clean --modcache`
* `go mod download`

**Expected behavior**
A clear and concise description of what you expected to happen.
Downloads artifacts at least at the same speed as the internet, but it also hangs when I am doing a download for a go module that I already built before through athens.

**Environment (please complete the following information):**
- OS: [e.g. Linux 64bit] Debian trixie amd64
- Go version : 1.26.0 (also fails with other versions)
- Proxy version : 0.18.1
- Storage (fs/mongodb/s3 etc.) :

**Additional context**
Add any other context about the problem here.

Reproduce the hang with `go clean --modcache` followed by `go mod download` using the provided go.mod and the reported Debian/Go 1.26.0 environment. Trace how Athens handles the `golang.org/toolchain` download and the 504 response from the configured proxy; done means the download completes without hanging, including for modules previously built through Athens.

Written by the indexing model from the issue text.