fix(security): block SSRF via the Gotenberg host setting (GHSA-mfxg) (#664)

gotenberg_host was validated only with Laravel's 'url' rule, which permits
loopback/private/link-local hosts (e.g. http://127.0.0.1, http://10.0.0.1,
the cloud metadata endpoint http://169.254.169.254). When a PDF renders, the
server POSTs the document HTML to that host — an SSRF primitive.

- Adds App\Rules\SafeRemoteUrl: requires http(s) and rejects any host that
  resolves to a loopback/private/link-local/CGNAT/reserved address (IPv4 and
  IPv6), including literal-IP hosts.
- Wires it into PDFConfigurationRequest for gotenberg_host.
- Adds a defensive re-check in GotenbergPDFDriver before the outbound call to
  cover hosts set via env/seed/stale config or DNS rebinding (TOCTOU).

Adds unit tests for the rule + validator integration.

app/Http/Requests/PDFConfigurationRequest.php

@@ -2,6 +2,7 @@
 
 namespace App\Http\Requests;
 
+use App\Rules\SafeRemoteUrl;
 use Illuminate\Foundation\Http\FormRequest;
 
 class PDFConfigurationRequest extends FormRequest
@@ -37,6 +38,7 @@ class PDFConfigurationRequest extends FormRequest
                     'gotenberg_host' => [
                         'required',
                         'url',
+                        new SafeRemoteUrl,
                     ],
                     'gotenberg_papersize' => [
                         'required',