[BUG] Missing signature URLs

Some releases do not have a signature URL:

```bash
curl "https://api.foojay.io/disco/v3.0/ids/9b0e6cc0427781fa76f4e5bba0579471"
```

```json
{
"result":[
{
"filename":"OpenJDK25U-jdk_ppc64_aix_hotspot_25.0.2_10.tar.gz",
"direct_download_uri":"https://github.com/adoptium/temurin25-binaries/releases/download/jdk-25.0.2%2B10/OpenJDK25U-jdk_ppc64_aix_hotspot_25.0.2_10.tar.gz",
"download_site_uri":"",
"signature_uri":"",
"checksum_uri":"",
"checksum":"191305cfb0d133b51c956a4ae007d468275f3ff0e9778ae0481336e29238071c",
"checksum_type":"sha256"
}
],
"message":""
}
```

However, I can see the signature here:

https://github.com/adoptium/temurin25-binaries/releases/tag/jdk-25.0.2%2B10

https://github.com/adoptium/temurin25-binaries/releases/download/jdk-25.0.2%2B10/OpenJDK25U-debugimage_ppc64_aix_hotspot_25.0.2_10.tar.gz.sig

as well as the checksum URI:

https://github.com/adoptium/temurin25-binaries/releases/download/jdk-25.0.2%2B10/OpenJDK25U-debugimage_ppc64_aix_hotspot_25.0.2_10.tar.gz.sha256.txt

I think they should be updated into the API as soon as they appear.
They exist since Jan 29, 2026.

Beginne damit, die Antwort vom API-Endpunkt `/disco/v3.0/ids/9b0e6cc0427781fa76f4e5bba0579471` zu reproduzieren und sie mit den verknüpften GitHub-Release-Assets zu vergleichen. Das Issue nennt keine Repository-Dateien oder Tests; abgeschlossen ist die Aufgabe, wenn Releases mit verfügbaren Signatur- und Prüfsummen-Assets die entsprechenden URIs in der API bereitstellen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.