ruff: add django rules

ruff.toml

@@ -5,6 +5,7 @@ select = [
     "A",       # flake8-builtins
     "B",       # flake8-bugbear
     "C4",      # flake8-comprehensions
+    "DJ",      # flake8-django
     "E",       # pycodestyle
     "F",       # pyflakes
     "G",       # flake8-logging-format
@@ -27,4 +28,9 @@ ignore = [
     "E731", # Do not assign a `lambda` expression, use a `def`
     "B904", # Within an `except` clause, raise exceptions with `raise ... from err`
     "RUF012", # Mutable class attributes should be annotated with `typing.ClassVar`
+    # TODO: add these one by one
+    "DJ001", # Avoid using `null=True` on string-based fields such as `CharField`
+    "DJ006", # Do not use `exclude` with `ModelForm`, use `fields` instead
+    "DJ008", # Model does not define `__str__` method
+    "DJ012", # Order of model's inner classes, methods, and fields does not follow the Django Style Guide: `Meta` class should come before `get_absolute_url`
 ]