Add invoice revision handling for repriced orders

[roncodes]

Summary

• make Ledger revision-aware when orders are repriced through a new purchase rate
• always create a new invoice for the new pricing revision instead of skipping invoice generation when one already exists
• update the FleetOps invoice tab to prefer the newest active invoice revision for an order

What Changed

• replaced the previous one-invoice-per-order guard in the purchase-rate observer
• voided prior draft invoices before creating the next invoice revision
• preserved sent and paid invoices as historical accounting records
• kept invoice creation sourced from the purchase rate event flow
• updated the order invoice component to fetch recent invoices and select the newest non-void, non-cancelled revision first

Verification

• php -l server/src/Observers/PurchaseRateObserver.php

Notes

• this PR is intentionally focused on Ledger revision behavior only
• style-only lint/prettier cleanup was intentionally left out of scope for this PR