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.

src/documents/tests/test_management_fuzzy.py

@@ -19,6 +19,10 @@ class TestFuzzyMatchCommand(TestCase):
         call_command(
             "document_fuzzy_match",
             "--no-progress-bar",
+            # The default scales with the CPU count and forks, which closes
+            # the test's database connection on PostgreSQL
+            "--processes",
+            "1",
             *args,
             stdout=stdout,
             stderr=stderr,