chore(server): disable indexer by default (#12710)

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

- **Chores**
	- Updated the default setting for the indexer feature to be disabled by default.
	- Added a sample environment variable for enabling the indexer in the example configuration file.
	- Introduced a new environment variable for the indexer in the CI workflow configuration.
- **Tests**
	- Adjusted test configurations to explicitly enable the indexer feature during test execution.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

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

@@ -30,7 +30,7 @@ declare global {
 defineModuleConfig('indexer', {
   enabled: {
     desc: 'Enable indexer plugin',
-    default: true,
+    default: false,
     env: ['AFFINE_INDEXER_ENABLED', 'boolean'],
   },
   'provider.type': {