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-front/src/modules/client-config/hooks/useClientConfig.ts

@@ -12,6 +12,8 @@ import { isConfigVariablesInDbEnabledState } from '@/client-config/states/isConf
 import { isDeveloperDefaultSignInPrefilledState } from '@/client-config/states/isDeveloperDefaultSignInPrefilledState';
 import { isClickHouseConfiguredState } from '@/client-config/states/isClickHouseConfiguredState';
 import { isCloudflareIntegrationEnabledState } from '@/client-config/states/isCloudflareIntegrationEnabledState';
+import { isDDLLockedState } from '@/client-config/states/isDDLLockedState';
+import { maintenanceModeState } from '@/client-config/states/maintenanceModeState';
 import { isEmailingDomainsEnabledState } from '@/client-config/states/isEmailingDomainsEnabledState';
 import { isEmailVerificationRequiredState } from '@/client-config/states/isEmailVerificationRequiredState';
 import { isGoogleCalendarEnabledState } from '@/client-config/states/isGoogleCalendarEnabledState';
@@ -117,6 +119,10 @@ export const useClientConfig = (): UseClientConfigResult => {
     isClickHouseConfiguredState,
   );
 
+  const setIsDDLLocked = useSetAtomState(isDDLLockedState);
+
+  const setMaintenanceMode = useSetAtomState(maintenanceModeState);
+
   const setAppVersion = useSetAtomState(appVersionState);
 
   const fetchClientConfig = useCallback(async () => {
@@ -195,6 +201,8 @@ export const useClientConfig = (): UseClientConfigResult => {
         clientConfig?.isCloudflareIntegrationEnabled,
       );
       setIsClickHouseConfigured(clientConfig?.isClickHouseConfigured ?? false);
+      setIsDDLLocked(clientConfig?.isWorkspaceSchemaDDLLocked ?? false);
+      setMaintenanceMode(clientConfig?.maintenance ?? null);
     } catch (err) {
       const error =
         err instanceof Error ? err : new Error('Failed to fetch client config');
@@ -229,7 +237,9 @@ export const useClientConfig = (): UseClientConfigResult => {
     setIsEmailingDomainsEnabled,
     setIsClickHouseConfigured,
     setIsCloudflareIntegrationEnabled,
+    setIsDDLLocked,
     setLabPublicFeatureFlags,
+    setMaintenanceMode,
     setIsMicrosoftCalendarEnabled,
     setIsMicrosoftMessagingEnabled,
     setSentryConfig,