feat(pdf): archival PDF/A output and document properties (#732)

Generated files carried no document properties at all, so an archive of them
showed a column of blank titles and no author. Title, Subject, Author and
Creator are now written from the document number and company, on both drivers:
dompdf via addInfo(), Gotenberg via metadata().

dompdf needed more than the API call. It reads Title from the <title> element
during render(), which happens after addInfo(), so metadata set through the API
alone was silently overwritten by whatever the template put there and the two
drivers disagreed about what the file was called. The title is written into the
markup as well, escaped.

Also adds an archival format setting for Gotenberg: off, PDF/A-1b, -2b or -3b.
PDF/A-3 is what the EU e-invoicing formats expect. Verified against a stock
gotenberg:8 -- LibreOffice inside the image does the conversion and the output
carries the right pdfaid:part in its XMP -- so no extra components are needed.

A fixed list rather than free text, because the SDK forwards whatever it is
given and an unsupported value would surface only as an HTTP error from the
service at render time. Empty is a real choice meaning an ordinary PDF, so it
overrides an env default rather than falling through it.

Gotenberg only: dompdf cannot produce PDF/A.

Claude-Session: https://claude.ai/code/session_01QmECndmNZwzN65Zz9P87dF

app/Http/Requests/PDFConfigurationRequest.php

@@ -10,6 +10,9 @@ use Illuminate\Validation\Rule;
 
 class PDFConfigurationRequest extends FormRequest
 {
+    /** Formats the Gotenberg image can actually produce, verified against gotenberg:8. */
+    public const PDFA_FORMATS = ['PDF/A-1b', 'PDF/A-2b', 'PDF/A-3b'];
+
     /**
      * Determine if the user is authorized to make this request.
      */
@@ -67,6 +70,10 @@ class PDFConfigurationRequest extends FormRequest
                 'url',
                 Rule::when(! $isDeclaredHost, [new PublicHttpUrl]),
             ],
+            // A fixed list rather than a free string: the SDK forwards whatever
+            // it is given, so an unsupported value would only fail later as an
+            // HTTP error from the Gotenberg service.
+            'gotenberg_pdfa' => ['nullable', Rule::in(self::PDFA_FORMATS)],
         ];
     }
 }