feat: build software licenses, purchase orders, vendors, and inventory stock modules

[Ibinola]

Summary of Changes

This pull request implements the backend modules assigned to ibinola:

• Consumable Inventory Module ([BE-93] Build the inventory module for consumable stock #1146): Created InventoryItem and StockMovement entities, supporting stock movement recording (POST /inventory/:id/movements), ledger balance calculation, and negative stock prevention.
• Vendors/Suppliers Module ([BE-94] Build the vendors/suppliers module #1147): Created Vendor entity, VendorsService, and VendorsController.
• Purchase Orders Module ([BE-95] Build the purchase orders module #1148): Created PurchaseOrder entity and receiving workflow (POST /purchase-orders/:id/receive).
• Software Licenses Module ([BE-96] Build the software licenses module for digital assets #1149): Created License entity and LicensesService enforcing total seat limits and redacting license keys in list responses.

Closes #1149
Closes #1148
Closes #1147
Closes #1146

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[yusuftomilola]

Reviewed the diff — matches its description. Adds an Inventory module (stock movements, ledger balance, negative-stock prevention), Vendors module, Purchase Orders module (with a receiving workflow), and a Licenses module (seat-limit enforcement, license-key redaction in list responses). All new, distinct file paths.

Same recurring app.module.ts additive-registration conflict as the rest of this batch; combined both sides since these are independent modules with no overlap with anything already merged. Resolved locally and pushed to main. Approving — this is the last of the 14 open PRs.