fix: prevent anonymous users from bypassing workspace creation restriction (#17635)

When `IS_WORKSPACE_CREATION_LIMITED_TO_SERVER_ADMINS` is true, anonymous
users could still create workspaces because
`checkWorkspaceCreationIsAllowedOrThrow` checked per-user workspace
count (always 0 for new users) instead of system-wide workspace count.

The bootstrap bypass now only applies when no workspaces exist in the
entire system. Also adds the same check in `signUpOnNewWorkspace` to
guard the `signInUp` code path.

Fixes #17631

Generated with [Claude Code](https://claude.ai/claude-code)

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>

packages/twenty-server/src/engine/core-modules/auth/services/sign-in-up.service.ts

@@ -403,8 +403,8 @@ export class SignInUpService {
     );
   }
 
-  private async isFirstWorkspaceForUser(userId: string): Promise<boolean> {
-    const count = await this.userWorkspaceService.countUserWorkspaces(userId);
+  private async isFirstWorkspaceInSystem(): Promise<boolean> {
+    const count = await this.workspaceRepository.count();
 
     return count === 0;
   }
@@ -414,7 +414,8 @@ export class SignInUpService {
   ): Promise<void> {
     if (!this.isWorkspaceCreationLimitedToServerAdmins()) return;
 
-    if (await this.isFirstWorkspaceForUser(currentUser.id)) return;
+    // Only allow bypass during initial system bootstrap (no workspaces exist yet)
+    if (await this.isFirstWorkspaceInSystem()) return;
 
     if (!currentUser.canAccessFullAdminPanel) {
       throw new AuthException(
@@ -445,6 +446,25 @@ export class SignInUpService {
       );
     }
 
+    if (
+      this.isWorkspaceCreationLimitedToServerAdmins() &&
+      !(await this.isFirstWorkspaceInSystem())
+    ) {
+      const isExistingAdmin =
+        userData.type === 'existingUser' &&
+        userData.existingUser.canAccessFullAdminPanel;
+
+      if (!isExistingAdmin) {
+        throw new AuthException(
+          'Workspace creation is restricted to admins',
+          AuthExceptionCode.FORBIDDEN_EXCEPTION,
+          {
+            userFriendlyMessage: msg`Workspace creation is restricted to admins`,
+          },
+        );
+      }
+    }
+
     const { canImpersonate, canAccessFullAdminPanel } =
       await this.setDefaultImpersonateAndAccessFullAdminPanel();