feat: depreciation engine, reservations, vendors directory, and purchase orders

[RennyThompson]

Summary

This PR implements four foundational features for the AssetsUp platform: a depreciation engine, asset reservations, a vendors directory, and purchase orders.

Changes

[BE-28] Depreciation Engine

• Added depreciationMethod, usefulLifeMonths, salvageValue, and currentValue fields to the Asset entity
• Created DepreciationModule with service supporting straight-line and declining-balance (200% DB) methods
• GET /api/assets/:id/depreciation — full depreciation schedule with current book value
• GET /api/reports/depreciation — org-wide totals (total book value, monthly expense, top categories)
• Handles mid-month purchases with prorated first periods, salvage value floor, and fully-depreciated assets

[BE-29] Asset Reservations

• Created ReservationsModule with Reservation entity (asset, reservedBy, time window, status)
• Race-safe overlap detection using raw SQL transaction checks
• POST /api/reservations — create with 409 on overlap
• GET /api/reservations — filtered list (assetId, userId, from, to, status)
• GET /api/assets/:id/availability — free/busy windows for calendar rendering
• PATCH /api/reservations/:id/cancel — staff cancel own; managers cancel any
• PATCH /api/reservations/:id/confirm — manager/admin only

[FE-06] Vendors Directory

• /vendors page with searchable vendor table (name, contact, email/phone, asset count)
• Create/edit modal with validation (email format, unique name/code)
• API client, TanStack Query hooks, and query keys
• Linked assets display on vendor detail

[FE-07] Purchase Orders

• /purchase-orders page with PO table (number, vendor, items, total, status badge, expected date)
• Create PO modal with vendor picker, dynamic line items, running total
• Receive flow: mark items received with quantities, auto-creates assets
• Status lifecycle: DRAFT → ORDERED → PARTIALLY_RECEIVED → RECEIVED, with cancel

Verification

• Backend modules follow existing NestJS patterns (TypeORM, service/controller/module)
• Frontend follows existing patterns (TanStack Query, React Hook Form + Zod, typed API clients)
• Sidebar updated with new navigation entries

Closes #1087
Closes #1088
Closes #1094
Closes #1095

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[yusuftomilola]

Reviewed. Solid work:

• Depreciation engine: straight-line and declining-balance methods, correctly proration on a partial first month, salvage-value floor respected, and both a per-asset and an org-wide category-grouped report.
• Reservations: overlap-conflict detection on create (409 on a clashing time window), ownership + role gating on cancel (owner/ADMIN/MANAGER), ADMIN/MANAGER-only confirm, and an availability endpoint wired into the asset controller.
• Purchase orders / vendors frontend: clean CRUD pages with search, receive/cancel modals, proper query hooks.

Approving.