Show what a backup contains before the operator types RESTORE

The restore dialog asked for its typed confirmation on nothing but a
filename and a date; the counts only surfaced in the audit row after the
rows were already replaced. A read-only summary endpoint now decrypts and
schema-validates the stored file through the same helpers the restore
itself runs and answers the summarizeBackup counts, and the dialog shows
them — per section, with an other-records line so a future summary key
can never silently vanish from the preview. A failed preview states its
own absence and never blocks the restore.

src/lib/query-keys/admin.ts

@@ -33,6 +33,9 @@ export const adminKeys = {
   adminModuleAvailability: () =>
     ["admin", "settings", "module-availability"] as const,
   adminBackups: () => ["admin", "backups"] as const,
+  /** v1.37.20 — restore preview: the counts a stored backup file carries. */
+  adminBackupSummary: (id: string) =>
+    ["admin", "backups", id, "summary"] as const,
   /** v1.23 — encryption coverage + rotation progress (admin Encryption section). */
   adminEncryptionStatus: () => ["admin", "encryption", "status"] as const,
   adminCoachFeedback: () => ["admin", "coach-feedback"] as const,