Gap 4: seed a throwaway database with PRODUCTION'S REAL SCHEMA SHAPE (table definitions only, obtained by the caller via a read-only pg_dump --schema-only against production -- this endpoint never touches production itself) BEFORE running the additive migration and the full rehearsal suite against it. Proves the new tables don't collide with any real existing table/constraint/index name and that init_projects_db() behaves identically alongside production's actual schema, not just an empty database.

Same REHEARSAL_DATABASE_URL-only safety rule as the sibling rehearsal endpoint. schema_sql is applied ONCE per call (the caller is expected to pass a fresh empty throwaway database each time -- this endpoint does not attempt to make the seed step itself idempotent, since a schema dump may contain plain CREATE TABLE, not CREATE TABLE IF NOT EXISTS).