Upload PDFs to the library, link them from any link target, show them in a viewer

PDFs go through the same upload field as images, are checked by their content and kept
unchanged in their own table. Every link field can pick one, and a page that links a PDF
opens it in a dialog with the browser's own viewer; touch devices and browsers without a
viewer get a new tab instead.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/TestSchema.php

@@ -21,6 +21,7 @@ use DoctrineMigrations\Version20260913000100;
 use DoctrineMigrations\Version20260914000100;
 use DoctrineMigrations\Version20260915000100;
 use DoctrineMigrations\Version20260916000100;
+use DoctrineMigrations\Version20260917000100;
 use Psr\Log\NullLogger;
 
 /**
@@ -53,6 +54,7 @@ final class TestSchema
             Version20260914000100::class,
             Version20260915000100::class,
             Version20260916000100::class,
+            Version20260917000100::class,
         ] as $migration) {
             (new $migration($db, new NullLogger()))->up($schema);
         }