action: leave torrent URLs empty in per-image asset manifests

The build action ships images, .asc and .sha files alongside its
artefacts but never produces a .torrent. Until now the manifest's
file_url_torrent and redi_url_torrent fields still got constructed by
appending '.torrent' to the file URL, so consumers of the manifest
saw URLs that always 404.

Return an empty string from signature_urls() for the torrent slot so
both file_url_torrent and redi_url_torrent are empty in the per-image
`<image>.assets.json`. The fields stay present for schema parity
with the canonical armbian-images.json index — if torrents ever start
being produced upstream, only the helper needs to change.

Author: igorpecovnik

action.yml

@@ -399,12 +399,16 @@ runs:
 
         # ── URL composition ──────────────────────────────────────────────────
         def signature_urls(base):
-            """Return (base, base.asc, base.sha, base.torrent)."""
-            return base, f"{base}.asc", f"{base}.sha", f"{base}.torrent"
+            """Return (base, base.asc, base.sha, ""). The build never
+            produces a .torrent for the assets it ships, so the torrent
+            slot is always empty — the field is kept for schema parity
+            with the canonical armbian-images.json index.
+            """
+            return base, f"{base}.asc", f"{base}.sha", ""
 
         def build_urls(parsed, image_format, img_name):
-            """Compose file_url + redi_url (each with its .asc/.sha/.torrent
-            siblings).
+            """Compose file_url + redi_url (each with its .asc/.sha
+            siblings; torrent slot is always empty — see signature_urls).
 
             Two URL shapes, selected by DOWNLOAD_REPO:
               non-empty -> {base}/{board}/{repo}/{filename}    dl.armbian.com style