Adds /api/admin/overview returning cached aggregate metrics for servers,
nodes, capacity, addresses, backups, and ISOs, plus a refreshed admin
dashboard UI consuming it via SWR.
Refactored from #153:
- Split OverviewService into focused per-section methods over a single
orchestrator, dropping the monolithic freshMetrics()
- Dropped the recent-activity panel and supporting index migration —
ActivityLog has no writers in the codebase, so the panel always
rendered empty
- Removed the 999.99 percent cap; overallocation is a legitimate state
and the frontend already clamps bar width
- Added capacity byte-conversion and restoring-bucket test cases
Co-Authored-By: Ajdin <AjdinDev@users.noreply.github.com>
