chore(server): add auto index batch size (#12391)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - Added a configurable batch size setting for automatic workspace indexing. Users can now adjust how many workspaces are indexed per batch, with a default value of 10 and support for values between 1 and 1000.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/admin/src/config.json

@@ -289,6 +289,10 @@
       "type": "String",
       "desc": "Indexer search service auth password, if not set, basic auth will be disabled. Optional for elasticsearch",
       "env": "AFFINE_INDEXER_SEARCH_PASSWORD"
+    },
+    "autoIndex.batchSize": {
+      "type": "Number",
+      "desc": "Number of workspaces automatically indexed per batch"
     }
   },
   "oauth": {