Turn on QuickBooks invoices and bills for a customer (#2642)

Runs the one-time subledger conversion on this company's Median books and only turns invoices and bills on when it passes: enter dual_read, convert (re-tags business-event journal entries' provenance; lines and amounts never change), validate INV-1..9 against a pre-conversion snapshot, stamp that validation, then set ar_ap_items_enabled. Idempotent and resumable: an enabled company returns already_enabled; a converted but unvalidated company resumes at validation. A failed or incomplete check returns 200 with outcome: validation_failed and the failing checks, and leaves the company in dual_read with invoices and bills off. Requires a verified QuickBooks connection. Never changes immediate AP journal entries or Stripe invoice sync. Takes no body.