Bulk create segments

Create up to 25 filter-subtype segments in one request. Each segment is created independently (own versions/1 snapshot); failures are reported per item in errors and do not roll back successful items. Returns 201 when at least one segment was created, 400 (same body shape) when every item failed. Use POST /v1/segments individually for matched/similarity/propensity segments. Requires 'creator' scope.

Each item's filters/filter_groups are validated against the live Field Catalog before it's created — an unknown field, a disallowed operator, or an invalid enum value fails that item only, reported in errors (same as any other per-item failure). If the Field Catalog itself is temporarily unreachable, the entire request fails with 502 FILTER_CATALOG_UNAVAILABLE rather than creating some segments unvalidated.