Carry the custom-metric entry tombstone through backup and restore

The entries gained a soft-delete this release; both backup ends follow in
the same one. A portable export omits tombstoned readings so a restore
cannot resurrect them; a disaster-recovery payload carries the tombstone
and the restore writes it back, so the account returns exactly as it
stood — undo affordance included.

src/lib/validations/backup.ts

@@ -611,6 +611,9 @@ const customMetricEntryBackupSchema = z
     measuredAt: isoDateTime,
     note: z.string().nullable().default(null),
     createdAt: isoDateTime.optional(),
+    /// v1.37.20 (A3-11) — the entry tombstone rides DR payloads so a restore
+    /// brings the account back exactly as it stood, undo affordance included.
+    deletedAt: isoDateTime.nullable().optional(),
   })
   .passthrough();