chore(readme): make the downloads badge live

[eneskirca]

Summary

• The README downloads badge was a hand-written 1.2k; the real installer count is ~12.8k (12,850 today: .dmg + .AppImage + .deb + .rpm + .exe across all releases).
• New .github/workflows/download-badge.yml recounts every 6 h, on each published release, and on manual dispatch. It writes a shields endpoint JSON (downloads.json) to an orphan badges branch as a single force-pushed commit. The file lives off main because main's required checks would block a bot commit there.
• The README badge now reads that file through img.shields.io/endpoint.
• Updater traffic (latest-*.yml polls, mac .zip deltas, blockmaps) is still excluded; shields' built-in github/downloads total reads ~14× higher because of it.
• A count of 0 or a failed read is refused, so a bad run never overwrites the real number.

After merge

The badges branch doesn't exist yet, so the badge renders as an error until the workflow runs once: Actions → Download badge → Run workflow.

Test plan

• Count query run against the live API (12,850)
• k/M label formatting checked (11472 → 11.5k, 12000 → 12k, 999 → 999, 1.5M)
• Workflow YAML parses
• First workflow_dispatch run after merge creates badges/downloads.json and the badge renders

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