onboard-spoke.sh: fix upstream construction for empty path with tag, replace fragile sed range with python3 for reliable union upstream append

hub/onboard-spoke.sh

@@ -163,12 +163,33 @@ if [[ "${ADD_UNION,,}" == "y" ]]; then
         3) UPSTREAM_TAG=":writeback" ;;
         *) warn "Invalid choice, defaulting to full read/write."; UPSTREAM_TAG="" ;;
     esac
+    if [ -n "$UNION_PATH" ]; then
         UPSTREAM="${SPOKE_NAME}-remote:${UNION_PATH}${UPSTREAM_TAG}"
+    else
+        UPSTREAM="${SPOKE_NAME}-remote:${UPSTREAM_TAG}"
+    fi
     if grep -q "^\[${UNION_NAME}\]" "$RCLONE_CONF" 2>/dev/null; then
         if grep -A5 "^\[${UNION_NAME}\]" "$RCLONE_CONF" | grep -qF "${SPOKE_NAME}-remote:"; then
             warn "Upstream for ${SPOKE_NAME}-remote already in union remote [${UNION_NAME}], skipping."
         else
-            sed -i "/^\[${UNION_NAME}\]/,/^\[/{s|^upstreams = \(.*\)|upstreams = \1 ${UPSTREAM}|}" "$RCLONE_CONF"
+            python3 - "$RCLONE_CONF" "$UNION_NAME" "$UPSTREAM" <<'PYEOF'
+import sys
+path, section, upstream = sys.argv[1], sys.argv[2], sys.argv[3]
+with open(path) as f:
+    lines = f.readlines()
+out = []
+in_section = False
+for line in lines:
+    if line.strip() == f"[{section}]":
+        in_section = True
+    elif line.strip().startswith("["):
+        in_section = False
+    if in_section and line.startswith("upstreams ="):
+        line = line.rstrip() + " " + upstream + "\n"
+    out.append(line)
+with open(path, "w") as f:
+    f.writelines(out)
+PYEOF
             info "Added '$UPSTREAM' to union remote [${UNION_NAME}]."
         fi
     else