feat(artifactapi): add open-webui/open-webui to ghcr immutable patterns (#173)

Part of #155 (prerequisite for open-webui deployment PR #172).

## Summary
- Adds `^open-webui/open-webui` to the `ghcr` remote's `immutable_patterns` in `remote-docker.yaml` so version-pinned open-webui image pulls are cached indefinitely through artifactapi

## Test plan
- artifactapi serves `ghcr.io/open-webui/open-webui:<version>` with `X-Artifact-Source: cache` on second fetch

Reviewed-on: #173

apps/base/artifactapi/resources/conf.d/remote-docker.yaml

@@ -6,6 +6,7 @@ remotes:
     immutable_patterns:
       - "^cloudnative-pg/cloudnative-pg"
       - "^emberstack/helm-charts"
+      - "^open-webui/open-webui"
       - "^openvoxproject/"
       - "^stakater/reloader"
       - "^stalwartlabs/stalwart"