Add cas_settlement_links table

so we can always answer “which settlement file touched this invoice” through SQL. Right now, that’s difficult to locate.

there're cases that invoice was reported as PAID, but later it marked NSF or something else on CAS side, which was difficult to reconcile. Having this mapping would give us clearer traceability and supporting evidence showing exactly which settlement files affected an invoice.

TODO: add table
reconciliation job update