fix(backup): drop the manifest key the records spread already carries

The assembly picked `manifest` out of the records section by hand, one line
under the comment explaining that both sections are spread precisely so nobody
has to pick their keys. It resolved to the same value the spread had already
placed, at the same position, so the emitted file is byte-identical either way.

Left in, it is the pattern that lost the custom cycle symptoms sitting next to
the note about how that happened.

src/lib/export/full-backup-payload.ts

@@ -369,7 +369,6 @@ export async function buildFullBackupPayload(
       unit: n.unit,
       source: n.source,
     })),
-    manifest: records.manifest,
   };
 
   return {