Add backup recovery center

docs/design/backup-recovery-center.md

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+# Backup recovery center
+
+## Problem
+
+An immediate undo button is not enough. A DJ may restart Serato Doctor, notice
+an issue later, or need to understand how much disk space safety snapshots use.
+
+## Architecture
+
+The recovery center reads the existing JSON manifests beneath
+`_Serato_/.serato-doctor-backups/`. It reports creation time, selected keeper,
+affected paths, restore status, and snapshot size. Invalid or incomplete backup
+folders are ignored rather than presented as restorable.
+
+Restore remains conservative: it only replaces a symbolic-link alias and
+refuses to overwrite a real file. A successful restore records its timestamp in
+the manifest so the UI cannot accidentally offer the same rollback twice.
+
+## Edge cases
+
+- No backup directory is treated as an empty history.
+- Malformed manifests do not prevent valid backups from loading.
+- Restored snapshots remain visible for audit purposes and disk accounting.
+- Backups can only be restored through the Serato library that owns them.
+
+## Tests
+
+- Backup history reports size and ready status.
+- Restore changes the persistent status to restored.
+- Static recovery assets are included in the package.