GST Transactions Importer

Upload a GST billing export (.csv or .zip). Rows with a domain in the description go into gst_transactions, mapped to a customer via domains → clients → email → as_customers (falling back to customer 108). Rows with no domain (handling fees, wallet top-ups) go into gst_fees instead. Matched by TRANSID — safe to re-run, already-imported rows are skipped.
Requires the tables from sql/create_gst_tables.sql to exist first.