Chore: make the backend test suite pass on PostgreSQL

Remove SQLite-only assumptions from the tests: hard-coded primary keys,
NULL ordering, the database vendor, and the CPU-dependent default process
count. Keep the test connection open inside the test transaction, since
close_old_connections() and connections.close_all() really close it on
PostgreSQL.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/documents/tests/test_bulk_edit.py

@@ -1031,9 +1031,7 @@ class TestPDFActions(DirectoriesMixin, TestCase):
         with self.assertLogs("paperless.bulk_edit", level="ERROR") as cm:
             bulk_edit.merge(doc_ids)
             error_str = cm.output[0]
-            expected_str = (
-                "Error merging document 2, it will not be included in the merge"
-            )
+            expected_str = f"Error merging document {self.doc2.id}, it will not be included in the merge"
             self.assertIn(expected_str, error_str)
 
         mock_consume_file.assert_not_called()
@@ -1193,7 +1191,7 @@ class TestPDFActions(DirectoriesMixin, TestCase):
         with self.assertLogs("paperless.bulk_edit", level="ERROR") as cm:
             bulk_edit.split(doc_ids, pages)
             error_str = cm.output[0]
-            expected_str = "Error splitting document 2"
+            expected_str = f"Error splitting document {self.doc2.id}"
             self.assertIn(expected_str, error_str)
 
         mock_consume_file.assert_not_called()