Export Cluster Run Data (CSV / Parquet)

One-click export of a clustering run's data — the exact rows behind the visualization (one row per member plus one per centroid: document id, cluster id and current label, x/y[/z] layout coordinates, per-cluster stats, and any custom LLM fields).

Formats:
- **`format=parquet`** (default): returns JSON with a short-lived
  presigned download URL for the run's `cluster_documents.parquet`
  artifact — the full-fidelity file (includes centroid vectors), any
  size. Download it promptly; the URL expires.
- **`format=csv`**: streams a spreadsheet-friendly CSV conversion —
  stable column order, current (renamed) cluster labels, no raw vectors.
  Capped at 100,000 rows; bigger runs get a clear 413 pointing to
  parquet. Scope to one cluster with `cluster_label` (accepts the run's
  cluster id like `cl_3` OR its current label).

Exports are strictly per-run: the run you pass is the run you get.
Runs that completed before artifacts existed, failed before the export
step, or whose artifacts have aged out of object storage return 404
with a message saying exactly what's missing.