ci: replace super-linter with ruff

Super-linter ran from the unpinned :latest image and, because of
branches-ignore: [master], never on pull requests to master. Ruff now
checks syntax errors and pyflakes rules (E9, F) on every push and PR.

Fixes the existing findings: unused imports and variables, an unused
duplicate import in instances/tests.py, and the star import in
accounts/views.py. settings-dev.py keeps its star import of the base
settings.

vrtManager/test_rwlock.py

@@ -128,7 +128,6 @@ class TestReadWriteLock(unittest.TestCase):
     def test_writer_blocks_subsequent_readers(self):
         """Pending writers take priority and should block subsequent new readers."""
         order = []
-        lock_held_event = threading.Event()
         writer_ready_event = threading.Event()
 
         # Step 1: Initial reader holds read lock