Use two-pass inserts for cyclic non-deferrable foreign keys

When two tables reference each other through non-deferrable foreign keys, backfill in two passes: insert the first table with its circular reference null, insert the referenced rows, then update the first table. Preserve parent-before-child order for every other dependency.

Large extracts should use keyset pagination and bounded chunks rather than one unbounded result. Validate foreign keys before and after the import, and do not infer safe scale from a single measured database.