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>

config/services.yaml

@@ -38,6 +38,9 @@ services:
     App\Content\MediaLibrary:
         arguments:
             $directory: '%env(projectdir:MEDIA_DIRECTORY)%'
+    App\Content\DocumentLibrary:
+        arguments:
+            $directory: '%env(projectdir:MEDIA_DIRECTORY)%'
     # Die Schriften liegen im oeffentlichen Ordner: sie sollen vom Webserver direkt ausgeliefert
     # werden, ohne PHP dazwischen - das ist der Punkt der Uebung.
     # Die Stellen, die wissen muessen, wie die Verwaltung heisst.