fix(security): block SSRF via the Gotenberg host setting (GHSA-mfxg) (#671)

v3 port. The Gotenberg PDF driver was missed when the SSRF guards were added
to the AI, exchange-rate and file-disk drivers: gotenberg_host was validated
only with 'url', and the driver POSTs the rendered HTML to it.

Reuses the existing infrastructure (consistency with the other drivers):
- Wires App\Rules\PublicHttpUrl into the gotenberg_host validation rule.
- Adds PrivateNetworkGuard::assertAllowed() in GotenbergPdfDriver before the
  outbound call (covers env/seed/stale config + DNS rebinding).

Adds a unit test asserting the gotenberg_host rule rejects private/loopback/
link-local addresses and allows a public one.

app/Http/Requests/PDFConfigurationRequest.php

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