ci: log HTTP status and response body on ISO upload failure

`curl -f` swallowed Gitea's error response, so the 3-attempt upload
failure on the 6330 MiB ISO reported nothing diagnosable. Capture the
status code via -w, print up to 2 KB of the response body for both
HTTP-level and transport-level failures, and clear upload.json at the
start of each attempt so a stale body cannot be misreported.

.gitea/workflows/build-iso.yaml

@@ -270,6 +270,9 @@ jobs:
             uploaded=0
             attempt=1
             while [ "$attempt" -le "$MAX_ATTEMPTS" ]; do
+              # Fresh output file each attempt: curl does not truncate -o on
+              # transport failure, so a stale body must not be misreported.
+              rm -f "$tmpdir/upload.json"
               # A previous attempt may have saved the asset but lost the
               # response; Gitea rejects duplicate names, so only after the
               # first attempt check for and remove same-name leftovers
@@ -292,19 +295,28 @@ jobs:
                 fi
               fi
 
-              if curl -sf -X POST --max-time 600 \
+              # Capture HTTP status explicitly: `-f` would swallow the Gitea
+              # error body (e.g. attachment size-limit messages) and leave the
+              # failure reason unknown.
+              if http_code=$(curl -sS -X POST --max-time 600 \
                   -H "Authorization: token $GITEA_TOKEN" \
                   -F "attachment=@$iso" \
                   "$GITEA_API/repos/$REPOSITORY/releases/$RELEASE_ID/assets" \
-                  -o "$tmpdir/upload.json"; then
-                if download_url=$(jq -er '.browser_download_url' "$tmpdir/upload.json"); then
+                  -o "$tmpdir/upload.json" -w '%{http_code}'); then
+                if [ "${http_code:0:1}" = "2" ] \
+                    && download_url=$(jq -er '.browser_download_url' "$tmpdir/upload.json"); then
                   echo "  ✅ Uploaded $fname (attempt $attempt/$MAX_ATTEMPTS): $download_url"
                   uploaded=1
                   break
                 fi
-                echo "  ⚠️ Upload of $fname returned no download URL (attempt $attempt/$MAX_ATTEMPTS)" >&2
+                echo "  ⚠️ Upload of $fname: HTTP $http_code (attempt $attempt/$MAX_ATTEMPTS)" >&2
+                echo "  Response: $(head -c 2000 "$tmpdir/upload.json" 2>/dev/null)" >&2
               else
-                echo "  ⚠️ Upload of $fname failed (attempt $attempt/$MAX_ATTEMPTS)" >&2
+                rc=$?
+                echo "  ⚠️ Upload of $fname failed (attempt $attempt/$MAX_ATTEMPTS): curl exit $rc, HTTP ${http_code:-000}" >&2
+                if [ -s "$tmpdir/upload.json" ]; then
+                  echo "  Response: $(head -c 2000 "$tmpdir/upload.json")" >&2
+                fi
               fi
 
               if [ "$attempt" -lt "$MAX_ATTEMPTS" ]; then