fix(settings): gate the AI settings page on AI_SETTINGS, not the chat flag (#21239)

## Summary

Closes #21229.

The two AI role permissions behaved **opposite to their labels**. The
trap is that the flag's code name is the inverse of its UI label:

| `PermissionFlagType` | UI label | Section | Means |
|---|---|---|---|
| `AI` | **"Ask AI"** | Actions | End-user: chat with AI |
| `AI_SETTINGS` | **"AI"** | Member / settings | Admin: configure AI
agents |

Before this PR (on `main`):
- `AI` ("Ask AI", chat) gated **both** the AI chat **and** the AI
settings page.
- `AI_SETTINGS` ("AI", configure agents) gated **nothing** the user
could see.

So a chat-only user could reach the whole AI **configuration** page, and
toggling the "AI" settings permission did nothing — exactly the
misalignment reported in #21229.

## Root cause

`PermissionFlagType.AI` *reads* like "the AI permission", so it looks
like the natural gate for the AI settings page — but it's actually the
**chat** flag. The settings page (nav item + route) had been pointed at
`AI` in #21072 to match the Overview stats query
(`findWorkspaceAiStats`), which was itself mis-gated on `AI`. Both the
stats query and the rest of the settings surface are admin/config
features, so they belong on `AI_SETTINGS`.

## Changes

All three move the **AI settings surface** from the chat flag (`AI`) to
the settings flag (`AI_SETTINGS`); chat keeps following `AI`:

- `useSettingsNavigationItems.tsx` — AI nav item → `AI_SETTINGS`
- `SettingsRoutes.tsx` — AI settings route group → `AI_SETTINGS`
- `ai-workspace-stats.resolver.ts` — `findWorkspaceAiStats`
(settings-only, drives the Overview tab) → `AI_SETTINGS`

After this: the "AI" permission controls the AI settings page + its
Overview; the "Ask AI" permission controls the chat. Both toggles now
match their labels.

## Test plan

- [ ] Role with **only "Ask AI"** (`AI`): AI chat tabs/pane visible;
**Settings → AI is hidden** and the route is not reachable.
- [ ] Role with **only "AI"** (`AI_SETTINGS`): Settings → AI is visible,
Overview stats load; chat nav is hidden.
- [ ] Admin (both flags): everything works as before.

## Known follow-ups (out of scope — pre-existing, shared endpoints)

These remain on `AI` because they're shared with non-settings surfaces
and need either OR-gating or a resolver split, so a role with
`AI_SETTINGS` but **not** `AI` still can't use them yet:

- `getAiSystemPromptPreview` (Models/Prompts tabs) lives in the chat
resolver, class-gated `AI`; NestJS guards are additive so it can't be
cleanly method-overridden — it should be pulled into a settings
resolver.
- Agent reads `findManyAgents` / `findOneAgent` (agent create/edit
forms) are class-gated `AI` and shared with the **Workflow** editor and
**Roles** pages; these want a guard that accepts `AI ∨ AI_SETTINGS ∨
WORKFLOWS`.

packages/twenty-front/src/modules/app/components/SettingsRoutes.tsx

@@ -690,7 +690,7 @@ export const SettingsRoutes = ({ isAdminPageEnabled }: SettingsRoutesProps) => (
       <Route
         element={
           <SettingsProtectedRouteWrapper
-            settingsPermission={PermissionFlagType.AI}
+            settingsPermission={PermissionFlagType.AI_SETTINGS}
           />
         }
       >

packages/twenty-front/src/modules/settings/hooks/useSettingsNavigationItems.tsx

@@ -171,7 +171,7 @@ const useSettingsNavigationItems = (): SettingsNavigationSection[] => {
           label: t`AI`,
           path: SettingsPath.AI,
           Icon: IconSparkles,
-          isHidden: !permissionMap[PermissionFlagType.AI],
+          isHidden: !permissionMap[PermissionFlagType.AI_SETTINGS],
         },
         {
           label: t`Email`,

packages/twenty-server/src/engine/metadata-modules/ai/ai-workspace-stats/resolvers/ai-workspace-stats.resolver.ts

@@ -13,7 +13,10 @@ import { WorkspaceAiStatsDTO } from 'src/engine/metadata-modules/ai/ai-workspace
 import { AiWorkspaceStatsService } from 'src/engine/metadata-modules/ai/ai-workspace-stats/services/ai-workspace-stats.service';
 import { UserRoleService } from 'src/engine/metadata-modules/user-role/user-role.service';
 
-@UseGuards(WorkspaceAuthGuard, SettingsPermissionGuard(PermissionFlagType.AI))
+@UseGuards(
+  WorkspaceAuthGuard,
+  SettingsPermissionGuard(PermissionFlagType.AI_SETTINGS),
+)
 @MetadataResolver()
 export class AiWorkspaceStatsResolver {
   constructor(