SQL DDL for the Threadline audit schema.
Used by mix threadline.install to generate a migration file. The generated
migration creates audit_transactions and audit_changes tables along with
the threadline_capture_changes() trigger function.
All DDL uses IF NOT EXISTS / CREATE OR REPLACE for idempotency (PKG-04).
Summary
Functions
Returns the full migration content as a string, ready to write to a .exs file.