Returns the per-contract snapshot lines of a bordereau GLI, in pagination-friendly form.
Each line is a frozen view captured at statement generation time — guarantees alignment with
what was effectively paid to the provider, even if the underlying contract was modified later.

Use this endpoint to:

1. Pay each cotisation accurately (Σ cotisation_in_cents == bordereau total)
2. Track exhaustive coverage (a contract absent here is not guaranteed for the next month)