Add workspace DDL lock env var and maintenance mode UI (#19130)

## Summary

- Add `WORKSPACE_SCHEMA_DDL_LOCKED` env-only boolean config variable
that blocks all workspace schema DDL changes when set to `true`. This is
intended for hot upgrades where logical replication cannot handle DDL
changes. Enforced at two chokepoints:
- `WorkspaceMigrationRunnerService.run` — blocks all metadata-driven DDL
(object/field/index CRUD, app sync/uninstall, standard app sync, upgrade
commands)
- `WorkspaceDataSourceService.createWorkspaceDBSchema` /
`deleteWorkspaceDBSchema` — blocks workspace creation (sign-up) and hard
deletion. Uses a dedicated `WorkspaceDataSourceException` (not
ForbiddenException)

- Add maintenance mode feature with Admin Panel UI and user-facing
banner:
- **Backend**: `MaintenanceModeService` stores maintenance window
(startAt, endAt, optional link) in `core.keyValuePair` as
`CONFIG_VARIABLE`. Validates endAt > startAt. Uses `GraphQLISODateTime`
scalar for date fields. Exposed via `clientConfig` REST endpoint and
admin GraphQL mutations (`setMaintenanceMode`, `clearMaintenanceMode`)
- **Admin Panel**: New "Maintenance Mode" section in Health tab with UTC
datetime pickers and activate/deactivate controls
- **Banner**: `InformationBannerMaintenance` displayed at the top of
`DefaultLayout` for all users, using Temporal API for timezone-aware
formatting with an optional "Learn more" link

These two features are **independent** — the DDL lock is controlled via
env var for operational use, while maintenance mode is a UI notification
mechanism controlled from the admin panel.

packages/twenty-server/src/engine/workspace-datasource/workspace-datasource.service.ts

@@ -7,12 +7,17 @@ import { FeatureFlagKey } from 'twenty-shared/types';
 import { type DataSource, type EntityManager, Repository } from 'typeorm';
 
 import { FeatureFlagService } from 'src/engine/core-modules/feature-flag/services/feature-flag.service';
+import { TwentyConfigService } from 'src/engine/core-modules/twenty-config/twenty-config.service';
 import { WorkspaceEntity } from 'src/engine/core-modules/workspace/workspace.entity';
 import { DataSourceService } from 'src/engine/metadata-modules/data-source/data-source.service';
 import {
   PermissionsException,
   PermissionsExceptionCode,
 } from 'src/engine/metadata-modules/permissions/permissions.exception';
+import {
+  WorkspaceDataSourceException,
+  WorkspaceDataSourceExceptionCode,
+} from 'src/engine/workspace-datasource/exceptions/workspace-datasource.exception';
 import { getWorkspaceSchemaName } from 'src/engine/workspace-datasource/utils/get-workspace-schema-name.util';
 
 @Injectable()
@@ -24,8 +29,19 @@ export class WorkspaceDataSourceService {
     private readonly coreDataSource: DataSource,
     private readonly featureFlagService: FeatureFlagService,
     private readonly dataSourceService: DataSourceService,
+    private readonly twentyConfigService: TwentyConfigService,
   ) {}
 
+  private assertDDLNotLocked(): void {
+    if (this.twentyConfigService.get('WORKSPACE_SCHEMA_DDL_LOCKED')) {
+      throw new WorkspaceDataSourceException({
+        message:
+          'Workspace schema DDL changes are locked. This is typically set during hot upgrades.',
+        code: WorkspaceDataSourceExceptionCode.DDL_LOCKED,
+      });
+    }
+  }
+
   public async checkSchemaExists(workspaceId: string) {
     const isDataSourceMigrated = await this.featureFlagService.isFeatureEnabled(
       FeatureFlagKey.IS_DATASOURCE_MIGRATED,
@@ -57,6 +73,8 @@ export class WorkspaceDataSourceService {
    * @returns
    */
   public async createWorkspaceDBSchema(workspaceId: string): Promise<string> {
+    this.assertDDLNotLocked();
+
     const schemaName = getWorkspaceSchemaName(workspaceId);
     const queryRunner = this.coreDataSource.createQueryRunner();
 
@@ -77,6 +95,8 @@ export class WorkspaceDataSourceService {
    * @returns
    */
   public async deleteWorkspaceDBSchema(workspaceId: string): Promise<void> {
+    this.assertDDLNotLocked();
+
     const schemaName = getWorkspaceSchemaName(workspaceId);
     const queryRunner = this.coreDataSource.createQueryRunner();