docs: add Tech Debt register and Inventory-on-Approval deep-dive
- New "Inventory on Approval" page: intended design (add ordered items to the
delivery site stock at approval), the NEXT_PUBLIC_INVENTORY_ENABLED flag
(UI-only gate), and how it actually behaves in prod -- dormant, because POs
carry no siteId under the vessel-only cost-centre model and the write is not
flag-gated. Includes remediation options.
- New "Tech Debt" register (Engineering section): TD-1 inventory-on-approval
dormant path; TD-2 migrations not coupled to the build (caused the paymentDate
P2022 prod incident).
- Sidebar links both pages; Inventory and Catalogue corrects the approval line
and cross-links the new pages.
