fix(accounts): make last-user removal atomic (lock + recount) to prevent a zero-user lock-out race

Codex P2: concurrent deletes could both pass the >1 check. Now the count + delete run in
one transaction under a row lock.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/Livewire/Settings/Users.php

@@ -6,6 +6,7 @@ use App\Models\AuditEvent;
 use App\Models\User;
 use App\Services\SessionService;
 use Illuminate\Support\Facades\Auth;
+use Illuminate\Support\Facades\DB;
 use Livewire\Attributes\On;
 use Livewire\Component;
 
@@ -57,19 +58,36 @@ class Users extends Component
     #[On('userRemoved')]
     public function remove(int $userId, SessionService $sessions): void
     {
-        $user = User::find($userId);
-        if (! $user || ! $this->canRemove($user)) {
+        $self = Auth::id();
+
+        // Atomic: lock the user rows + recount inside the transaction so two concurrent
+        // deletes can't both pass the "more than one account" check and strand the
+        // install with zero users (a lock-out). The last account is never removable.
+        $email = DB::transaction(function () use ($userId, $self, $sessions): ?string {
+            if (User::lockForUpdate()->count() <= 1) {
+                return null;
+            }
+
+            $user = User::find($userId);
+            if (! $user || $user->getKey() === $self) {
+                return null;
+            }
+
+            $email = $user->email;
+
+            // Kill every session + rotate the remember_token BEFORE deleting, so a
+            // stolen remember-me cookie can never resurrect the account between the
+            // session wipe and the row delete.
+            $sessions->logoutUserEverywhere($user);
+            $user->delete();
+
+            return $email;
+        });
+
+        if ($email === null) {
             return;
         }
 
-        $email = $user->email;
-
-        // Kill every session + rotate the remember_token BEFORE deleting, so a
-        // stolen remember-me cookie can never resurrect the account between the
-        // session wipe and the row delete.
-        $sessions->logoutUserEverywhere($user);
-        $user->delete();
-
         $this->audit('user.delete', $email);
         $this->dispatch('notify', message: __('accounts.remove_notify'));
     }