style(admin): rename artifact thumbnail field label to "PDF thumbnail" (#1380)

Per review feedback, "Thumbnail preview" → "PDF thumbnail" on the change form
(clearer that it's the PDF-derived thumbnail). Also updates the <img> alt text
and the regression test to match.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jonfroehlich

website/admin/artifact_admin.py

@@ -89,12 +89,13 @@ def thumbnail_preview(self, obj):
             )
             return placeholder
         return format_html(
-            '<img src="{}" alt="Thumbnail preview" '
+            '<img src="{}" alt="PDF thumbnail" '
             'style="height:{}px; width:auto; border:1px solid #ddd;" />',
             thumbnail_url, self.THUMBNAIL_PREVIEW_HEIGHT,
         )
 
-    thumbnail_preview.short_description = 'Thumbnail preview'
+    # Django auto-appends the trailing colon in the admin label.
+    thumbnail_preview.short_description = 'PDF thumbnail'
 
     def get_fieldsets(self, request, obj=None):
         """

website/tests/test_admin_thumbnail_preview.py

@@ -112,7 +112,7 @@ def test_change_form_get_renders_img_end_to_end(self):
 
         self.assertEqual(resp.status_code, 200)
         body = resp.content.decode()
-        self.assertIn('alt="Thumbnail preview"', body)
+        self.assertIn('alt="PDF thumbnail"', body)
         self.assertIn("<img", body)
         # The tag is real markup, not escaped into visible text.
         self.assertNotIn("&lt;img", body)