Persist dashboard backup metrics separately

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+### 2026.07.13.r009
+- Store completed-backup chart measurements independently in `backup-metrics.json` so clearing activities no longer erases the dashboard history.
+- Migrate existing successful backup runs into the new durable metric history automatically and expose them through a dedicated read-only API.
+- Keep up to 5,000 compact measurements and clarify in the delete dialog that dashboard chart data is preserved.
+
 ### 2026.07.13.r008
 - Remove the nested vertical scrollbar from the snapshot repository browser and use the normal Unraid page scroll.
 - Keep lazy folder loading and cap deep tree indentation so long paths remain usable on desktop and mobile.