feat: implement admin overview dashboard with metrics and API endpoint

[AjdinDev]

This pull request introduces a new admin dashboard overview endpoint that provides real-time metrics and recent activity for administrators. It includes the backend API, data aggregation and transformation logic, frontend API integration, and supporting database and localization changes. The implementation is covered by feature tests and includes performance improvements via new database indexes.

Backend API and Data Aggregation:

• Added a new OverviewController and OverviewService to aggregate and serve admin dashboard metrics, including server statuses, resource allocation, address usage, backup and ISO statistics, per-node metrics, and recent activity. Results are cached for 15 seconds for efficiency.
• Implemented OverviewTransformer to format the aggregated metrics for API responses, ensuring consistent and structured output.
• Registered the new API route at /api/admin/overview for accessing the overview data.

Frontend Integration:

• Added TypeScript interfaces and data transformation logic in getOverview.ts to consume and type the new API endpoint in the admin dashboard frontend.
• Introduced a SWR hook useOverviewSWR.ts for efficient, auto-refreshing data fetching on the dashboard (refreshes every 10 seconds).

Performance and Localization:

• Added database indexes on the activity_logs.timestamp and servers.status columns to improve query performance for dashboard metrics.
• Added English localization strings for the dashboard overview panel and its UI elements.

Testing:

• Added feature tests to verify correct metrics aggregation, API response structure, and admin-only access restrictions for the new endpoint.

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