Follow-up to #3675 and #3709.

PR #3709 correctly hardened packaging/linux/io.github.aethersdr.aethersdr.metainfo.xml for Flathub submission, but the <releases> block and the screenshot URL are now hand-maintained point-in-time values. Every future release requires a manual PR to:

• Prepend a new <release version="…" date="…"/> entry and trim the list to N recent releases
• Update the screenshot URL from the previous tag to the new one

Issue #3675 already anticipated this: "Could later be auto-generated from tags in CI."

Proposed solution

A GitHub Actions workflow triggered on push of a v* tag that:

1. Parses the tag name and tag date
2. Prepends a new <release> entry to the <releases> block and trims to the last 5 (or configurable N)
3. Updates the screenshot <image> URL to point at the new tag
4. Commits the updated file back to main (or opens a draft PR for maintainer review)

Scope

One workflow file in .github/workflows/ plus optionally a small helper script. No source code changes. The metainfo file itself needs no structural changes — just the two dynamic sections.

Notes

• The <release date> attribute should use the git tag date (tagger timestamp), not the GitHub release publication date, to stay consistent with the convention established in chore(metainfo): polish AppStream for Flathub submission #3709 (PDT local time)
• The screenshot URL pattern established in chore(metainfo): polish AppStream for Flathub submission #3709 (https://raw.githubusercontent.com/aethersdr/AetherSDR/<tag>/docs/assets/screenshot-current.png) is the correct template to parameterise