Reduce attachment migration batch size to 1

This prevents ‘Maximum IPC message size exceeded’ due to IDB `getAll` operation.

- https://github.com/zincbase/zincdb/issues/17
- https://cs.chromium.org/chromium/src/content/browser/indexed_db/indexed_db_database.cc?q=%22Maximum+IPC+message+size+exceeded%22&sq=package:chromium&l=1160

js/modules/messages_data_migrator.js

@@ -19,7 +19,7 @@ const { deferredToPromise } = require('./deferred_to_promise');
 
 
 const MESSAGES_STORE_NAME = 'messages';
-const NUM_MESSAGES_PER_BATCH = 50;
+const NUM_MESSAGES_PER_BATCH = 1;
 
 exports.processNext = async ({
   BackboneMessage,