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/backend/server/src/plugins/indexer/config.ts

@@ -31,6 +31,7 @@ defineModuleConfig('indexer', {
   enabled: {
     desc: 'Enable indexer plugin',
     default: true,
+    env: ['AFFINE_INDEXER_ENABLED', 'boolean'],
   },
   'provider.type': {
     desc: 'Indexer search service provider name',