feat(security): add session timeout + transaction wrapping (B1)

Session timeout: configurable idle (default 30min) and absolute (default 8h)
timeouts via DB settings, enforced in web/index.php on every request.
Timestamps set at login in action layer; graceful fallback for pre-existing
sessions.

Transaction wrapping: UserAccountService.createFromAdmin/register, roles
create/edit, and permissions edit now wrap multi-step DB operations in
transactions to guarantee atomicity.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pages/auth/login().php

@@ -312,6 +312,8 @@ if (requestInput()->method() === 'POST') {
                                             if ($remember) {
                                                 $rememberMeService->rememberUser($userId);
                                             }
+                                            $sessionStore->set('session_started_at', time());
+                                            $sessionStore->set('session_last_activity', time());
                                             Router::redirect('admin');
                                             return;
                                         }