Fix: stop full document saves from writing file path columns

A full save() of a Document instance loaded before another process moved its file wrote the old filename back. The DB then pointed at a path that no longer existed, and recovery only worked when the recomputed path happened to match the file's real location.

Document.save() now leaves filename and archive_filename out of full saves on existing rows. Writers that own those columns opt in with write_file_paths=True or an explicit update_fields. A full save of a row deleted elsewhere now raises instead of re-inserting it.

src/documents/tests/test_bulk_edit.py

@@ -739,7 +739,7 @@ class TestPDFActions(DirectoriesMixin, TestCase):
             mime_type="application/pdf",
         )
         self.doc1.archive_filename = sample1_archive
-        self.doc1.save()
+        self.doc1.save(write_file_paths=True)
         self.doc2 = Document.objects.create(
             checksum="B",
             title="B",
@@ -749,7 +749,7 @@ class TestPDFActions(DirectoriesMixin, TestCase):
             created=date(2023, 1, 2),
         )
         self.doc2.archive_filename = sample2_archive
-        self.doc2.save()
+        self.doc2.save(write_file_paths=True)
         self.doc3 = Document.objects.create(
             checksum="C",
             title="C",
@@ -774,7 +774,7 @@ class TestPDFActions(DirectoriesMixin, TestCase):
             created=date(2023, 1, 3),
         )
         self.img_doc.archive_filename = img_doc_archive
-        self.img_doc.save()
+        self.img_doc.save(write_file_paths=True)
 
     @staticmethod
     def fake_decrypt(