v1.3.2 , the subscriber export finishes

export_subscribers failed on every call. It works now.

Substack answers 400 {"error":"Export not ready"} while it generates the file — for
about three seconds even on a six-row export. The first poll went out immediately, read
that refusal as fatal and gave up, so every export ended the same way:

export_subscribers({filters: […], columns: […]})
  → SubstackAPIException: 400 Bad Request              (every call, up to 1.3.1)
  → {count: 6, columns: […], subscribers: […]}         (1.3.2)

The poll now reads that one answer as "still working" and keeps to its wait budget. Any
other refusal still fails at once, instead of polling until the budget runs out.

This affected every release since v1.1.0, where the tool first shipped. Nothing else
used that code path, so no other tool was involved.

Also in this release

• group_membership exports after all. The tool used to announce it as one of two
  columns that cannot be exported; only tag_ids is. If you left that column out
  because the description said to, ask for it.
• A failing request now carries Substack's own explanation of the refusal, not just the
  status code. The original failure reported 400 Bad Request and nothing else, which
  named neither the step that failed nor the reason — and cost a wrong diagnosis.

No inputs changed. If your calls were failing, they need no edit; if you had given up
on this tool, it is worth another try.

Reported in #25.