[BE-67] Build Audit Log viewer endpoint — searchable, filterable history of all system changes

[yusuftomilola]

Overview

While AssetHistory records exist for asset-level changes, there is no unified audit trail covering all entity types (users, departments, roles, locations). Compliance and security teams need a single endpoint to query what changed, who changed it, and when.

Context

• An AuditLog entity may partially exist from prior PRs — verify before creating a new one
• Every write operation across all modules should append an audit record
• The endpoint must support pagination and multiple filter dimensions simultaneously

Acceptance Criteria

• Create (or verify) AuditLog entity: id, entityType (e.g., asset, user, department), entityId, action (CREATED, UPDATED, DELETED, STATUS_CHANGED, ROLE_CHANGED), actorId (FK → User), previousValue (JSON), newValue (JSON), ipAddress, userAgent, createdAt
• Create an AuditService.log() method and call it from AssetsService, UsersService, DepartmentsService, and AuthService on every mutating operation
• GET /audit-logs (ADMIN only): accepts query params — entityType, entityId, actorId, action, from (date), to (date), page, limit
• GET /audit-logs/:id — returns full detail of a single log entry including the JSON diff
• Response includes actor's firstName and lastName (not just ID)
• Apply caching with a short TTL (30 seconds) on the list endpoint