ci: make release step idempotent + fix Gitea asset upload

- resolve-or-create the release by tag to avoid the 409 'Release is has no Tag'
  when a release already exists for the tag (e.g. one made manually in the UI)
- use POST /releases/{id}/assets with multipart field 'attachment'
  (was the obsolete /_attachments route with field 'file')

.github/workflows/release.yml

@@ -42,13 +42,40 @@ jobs:
             exit 0
           fi
 
-          release_id=$(curl -s -X POST \
+          json_id() { node -e 'let d="";process.stdin.on("data",c=>d+=c).on("end",()=>{const j=JSON.parse(d||"{}");if(j.id==null){console.error("no release id:",JSON.stringify(j));process.exit(1)}console.log(j.id)})'; }
+
+          release_id=$(
+            curl -s -o /tmp/rel.json -w "%{http_code}" \
+              -H "Authorization: token $GITEA_TOKEN" \
+              "$GITEA_API/repos/$REPO/releases/tags/$TAG"
+          )
+
+          if [ "$release_id" = "200" ]; then
+            echo "Release for $TAG already exists; reusing it."
+            release_id=$(json_id < /tmp/rel.json)
+          else
+            echo "Creating release for $TAG."
+            release_id=$(
+              curl -s -o /tmp/rel.json -w "%{http_code}" -X POST \
                 -H "Authorization: token $GITEA_TOKEN" \
                 -H "Content-Type: application/json" \
                 -d "{\"tag_name\": \"$TAG\", \"name\": \"$TAG\", \"body\": \"Automated release build for $TAG\"}" \
-            "$GITEA_API/repos/$REPO/releases" | node -e 'let d="";process.stdin.on("data",c=>d+=c).on("end",()=>{const j=JSON.parse(d);if(j.id==null){console.error("release create failed:",JSON.stringify(j));process.exit(1)}console.log(j.id)})')
+                "$GITEA_API/repos/$REPO/releases"
+            )
+            case "$release_id" in
+              201|200) release_id=$(json_id < /tmp/rel.json) ;;
+              *) echo "::error::release create returned HTTP $release_id: $(cat /tmp/rel.json)"; exit 1 ;;
+            esac
+          fi
 
-          curl -s -X POST \
+          echo "Uploading asset to release $release_id."
+          resp=$(curl -s -o /tmp/att.json -w "%{http_code}" -X POST \
              -H "Authorization: token $GITEA_TOKEN" \
-            -F "file=@vidplex-${TAG}.zip" \
-            "$GITEA_API/repos/$REPO/releases/$release_id/_attachments"
+             -F "attachment=@vidplex-${TAG}.zip" \
+             "$GITEA_API/repos/$REPO/releases/$release_id/assets")
+          if [ "$resp" != "200" ] && [ "$resp" != "201" ]; then
+            echo "::error::asset upload returned HTTP $resp: $(cat /tmp/att.json)"
+            exit 1
+          fi
+          cat /tmp/att.json
+          echo