Add a row to a data table

Adds one row of master data to a table. data is keyed by column key and must satisfy the table’s own columns, so call tabelle_get_table first — a key it does not declare is refused, and so is a value of the wrong type. It does NOT de-duplicate: two rows sharing the value in a flow’s label column collapse into one option and only the first row’s other columns are ever read, so search before you add. At most 5000 rows per table, and only the first 500 ever become a dropdown’s options.