feat: Inventory management, admin audit log backend, and promo code checkout integration

[Thommyy7]

Summary

This PR implements four features assigned to Thommyy7:

[BE-28] Inventory Management: Backend (#1203)

• New InventoryItem entity: name, category, serialNumber (nullable, unique), location, condition (GOOD/FAIR/NEEDS_REPAIR/RETIRED), assignedToUserId (nullable FK), assignedAt, purchasedAt, notes, isDeleted (soft delete)
• POST /inventory — add item (admin only)
• PATCH /inventory/:id — update details, condition, or location (admin only)
• DELETE /inventory/:id — soft-delete / retire item (admin only; only if unassigned)
• GET /inventory — list items (admin/staff, filterable by category, condition, location, assignedToUserId)
• POST /inventory/:id/assign — assign to member (admin only)
• POST /inventory/:id/unassign — release from member (admin only)
• InventoryModule registered in AppModule

[FE-25] Inventory Management: Frontend (#1204)

• /admin/inventory page with data table: Name, Category, Serial No., Location, Condition badge (colour-coded), Assigned To
• Condition badge colours: green (Good), amber (Fair), red (Needs Repair), grey (Retired)
• Category and condition filter dropdowns
• Add Item button → form modal with all required fields
• useGetInventory, useCreateInventoryItem, useUpdateInventoryItem, useAssignInventoryItem React Query hooks

[BE-29] Admin Audit Log: Backend (#1205)

• New AuditLog entity: actorUserId, action, resourceType, resourceId, metadata (JSONB), ipAddress, createdAt (immutable records — no update/delete endpoints)
• AuditLogService.log() method for use by other services
• GET /audit-log — paginated, filterable by actorId, resourceType, startDate, endDate (super_admin only)
• AuditLogModule registered in AppModule and exported for use by other modules

[FE-24] Discount / Promo Codes: Frontend - Checkout Integration (#1202)

• "Have a promo code?" collapsible field added to BookingForm below the price estimate
• Apply button calls POST /promo-codes/validate; on success shows discount line (Discount: -₦X) and updates displayed final amount
• Inline error messages for invalid/expired/limit-reached codes
• Remove link clears the applied code and restores original price
• promoCode state tracked in form for future POST /bookings body integration

Closes #1203
Closes #1204
Closes #1205
Closes #1202

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