POST /api/transform/extract-columns

Extract column names suitable for incremental transform checkpoint filtering.

This endpoint is specifically for populating the checkpoint column dropdown in incremental transforms. It only returns columns with types supported for checkpoint filtering: temporal (timestamp/tz) and numeric (int/float) types.

Text, boolean, and other unsupported column types are filtered out.

The query is compiled to native SQL using [[qp.compile/compile-with-inline-parameters]], which handles parameterized queries with template tags. Then extracts column names and types using PreparedStatement metadata.

Returns a map with a :columns key containing a vector of column names (strings). If extraction fails, returns nil for :columns.