chore(server): support disable indexer plugin (#12408)

close CLOUD-220

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

## Summary by CodeRabbit

- **New Features**
  - Introduced a new service to handle indexing-related events and scheduled tasks, improving the management of document and workspace indexing.
  - Added support for configuring the indexer feature via the AFFINE_INDEXER_ENABLED environment variable.

- **Bug Fixes**
  - Ensured that indexing and deletion jobs are only enqueued when the indexer feature is enabled.

- **Tests**
  - Added comprehensive tests for the new indexing event service, covering various configuration scenarios.
  - Removed obsolete test related to auto-indexing scheduling.

- **Chores**
  - Updated configuration descriptions and mappings to improve clarity and environment variable support.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/admin/src/config.json

@@ -267,7 +267,8 @@
   "indexer": {
     "enabled": {
       "type": "Boolean",
-      "desc": "Enable indexer plugin"
+      "desc": "Enable indexer plugin",
+      "env": "AFFINE_INDEXER_ENABLED"
     },
     "provider.type": {
       "type": "String",