OpenURI::HTTPError: 404 Not Found (OpenURI::HTTPError)

## Error

```
404 Not Found (OpenURI::HTTPError)
```

## Stack trace

```
open-uri.rb:393 in OpenURI.open_http
open-uri.rb:825 in buffer_open
open-uri.rb:233 in block in OpenURI.open_loop
open-uri.rb:231 in catch
open-uri.rb:231 in OpenURI.open_loop
open-uri.rb:163 in OpenURI.open_uri
open-uri.rb:805 in open
app/commands/user/avatar_data_uri.rb:60 in download_external_avatar
app/commands/user/avatar_data_uri.rb:54 in external_data_uri
app/commands/user/avatar_data_uri.rb:37 in call
```

## Tracking Details
- **Project**: exercism-rails
- **Sentry**: [OpenURI::HTTPError: 404 Not Found (OpenURI::HTTPError)](https://thalamus-ai.sentry.io/issues/143283031/)
- **Culprit**: SPI::SolutionImageDataController#show
- **First seen:** Aug 27, 2026

Start with app/commands/user/avatar_data_uri.rb, especially download_external_avatar at line 60, and trace the call from external_data_uri and call. The issue identifies a 404 from OpenURI but does not state the expected handling or completion criteria, so those need to be clarified before implementation.

Written by the indexing model from the issue text.