RSS feed GUIDs are unstable when multi-architecture wheels arrive at different times

Summary

The GUID fix from #1374 (appending #timestamp to make rebuilds visible) causes feed readers to show duplicate entries when platform-specific wheels for the same (name, version, build_tag) arrive at different times.

Root cause

iter_releases() groups by (name_normalized, version) and annotates added_at=Max("file_added_at"). The GUID is link#pubdate.isoformat(). When a new architecture wheel is promoted (e.g., s390x arriving an hour after x86_64/aarch64/ppc64le), Max(file_added_at) shifts, producing a different GUID. Feed readers treat the new GUID as a new item.

Proposed fix

Replace the timestamp-based GUID fragment with a build-tag-based fragment. Use ArrayAgg("filename") in the query, extract PEP 427 build tags from wheel filenames in Python, and construct GUIDs like link#builds=1,2.

This makes GUIDs:

• Stable when additional platform wheels arrive for the same build tag
• Different when a genuine rebuild introduces a new build tag (the original #1374 requirement)

Start at iter_releases(), where releases are grouped and annotated with file_added_at, and trace how the RSS GUID uses link and pubdate. Update the query and GUID construction according to the proposed build-tag approach, then verify that later platform wheels keep the same GUID while a new build tag changes it.