Improve security, validation, and isolation checks

Add multiple security and validation improvements across the app:

- Prevent session fixation: regenerate session ID on login and after successful 2FA; tighten session cookie params (Secure, HttpOnly, SameSite=Lax).
- Harden installer: add CSRF checks for install/update flows and use PDO::quote when injecting admin credentials into SQL migration to avoid injection; add csrf_field() to installer templates.
- Template hardening: add safe_url and safe_mailto Twig filters, escape tag names for JS, and add rel="noopener noreferrer" to external links to mitigate XSS/opener risks.
- Domain controller: validate referrer to avoid open redirects, enforce user isolation mode when finding/deleting/updating domains and when assigning notification groups (ensures users only affect their own resources).
- Notification groups: verify channel belongs to group before deleting or toggling to prevent unauthorized access.
- ErrorLog: whitelist allowed sort columns to avoid arbitrary column injection in ORDER BY.
- Routes: move the debug whois route to protected/admin area.

These changes collectively reduce attack surface (XSS, open redirect, session fixation, SQL injection) and enforce proper resource isolation and input validation.

app/Controllers/NotificationGroupController.php

@@ -687,6 +687,13 @@ class NotificationGroupController extends Controller
             return;
         }
 
+        $channel = $this->channelModel->find($id);
+        if (!$channel || (int)$channel['notification_group_id'] !== (int)$groupId) {
+            $_SESSION['error'] = 'Channel not found';
+            $this->redirect("/groups/$groupId/edit");
+            return;
+        }
+
         try {
             $this->channelModel->delete($id);
             $_SESSION['success'] = 'Channel deleted successfully';
@@ -714,6 +721,13 @@ class NotificationGroupController extends Controller
             return;
         }
 
+        $channel = $this->channelModel->find($id);
+        if (!$channel || (int)$channel['notification_group_id'] !== (int)$groupId) {
+            $_SESSION['error'] = 'Channel not found';
+            $this->redirect("/groups/$groupId/edit");
+            return;
+        }
+
         try {
             $this->channelModel->toggleActive($id);
             $_SESSION['success'] = 'Channel status updated';