backfill widget position from gridPosition (phase 1 of gridPosition removal) (#20032)

## Context
Phase 1 of removing the legacy `gridPosition` field from
`PageLayoutWidget` in favor of the new `position` discriminated union
(`grid` / `vertical-list` / `canvas`). This PR is purely additive —
`gridPosition` is still required and read everywhere; we just guarantee
that every widget now also has a non-null `position` so a follow-up PR
can drop `gridPosition` cleanly.

## Changes
- **Slow instance command**
`BackfillPageLayoutWidgetPositionSlowInstanceCommand` (2.1.0):
for every `core.pageLayoutWidget` row where `position IS NULL`, copies
`gridPosition`
into `position` with `layoutMode: 'GRID'`. Historically only grid
widgets used
`gridPosition`, so a single SQL update covers every existing row.
- **`PageLayoutDuplicationService`**: when duplicating a widget, also
forwards
`originalWidget.position` (was previously only forwarding
`gridPosition`).
- **Frontend default layouts** (10 `Default*PageLayout.ts` files): added
a `position`
sibling to every widget, matching the parent tab's `layoutMode` —
`VERTICAL_LIST` widgets
get `{ layoutMode, index }`, `CANVAS` widgets get `{ layoutMode }`

<img width="338" height="226" alt="Screenshot 2026-04-24 at 16 23 17"
src="https://github.com/user-attachments/assets/c3319318-f1b8-4271-96b4-196b209a1f5e"
/>

packages/twenty-server/src/database/commands/upgrade-version-command/instance-commands.constant.ts

@@ -16,6 +16,7 @@ import { AddGlobalObjectContextToCommandMenuItemAvailabilityTypeFastInstanceComm
 import { AddPageLayoutIdToCommandMenuItemFastInstanceCommand } from 'src/database/commands/upgrade-version-command/1-23/1-23-instance-command-fast-1776168404836-add-page-layout-id-to-command-menu-item';
 import { AddConditionalAvailabilityExpressionToPageLayoutWidgetFastInstanceCommand } from 'src/database/commands/upgrade-version-command/1-23/1-23-instance-command-fast-1775654781000-add-conditional-availability-expression-to-page-layout-widget';
 import { AddIsPreInstalledToApplicationRegistrationFastInstanceCommand } from 'src/database/commands/upgrade-version-command/2-0/2-0-instance-command-fast-1776886452831-add-is-pre-installed-to-application-registration';
+import { BackfillPageLayoutWidgetPositionSlowInstanceCommand } from 'src/database/commands/upgrade-version-command/2-1/2-1-instance-command-slow-1795000002000-backfill-page-layout-widget-position';
 
 export const INSTANCE_COMMANDS = [
   AddViewFieldGroupIdIndexOnViewFieldFastInstanceCommand,
@@ -34,4 +35,5 @@ export const INSTANCE_COMMANDS = [
   AddPageLayoutIdToCommandMenuItemFastInstanceCommand,
   AddConditionalAvailabilityExpressionToPageLayoutWidgetFastInstanceCommand,
   AddIsPreInstalledToApplicationRegistrationFastInstanceCommand,
+  BackfillPageLayoutWidgetPositionSlowInstanceCommand,
 ];