feat: include migration storage in system overview

[Willxup]

Summary

• include migration-directory storage in the system overview using the existing cached migration snapshot
• group persistent-data and runtime metrics into two responsive four-metric cards
• expose Go Heap Sys and keep Goroutine as the final runtime metric
• update the OpenAPI contract, generated types, translations, and coverage

Verification

• make check
• verified the updated metric order on the local /admin/system page