ruff: enable B (flake8-bugbear)

Fixes 71 violations across production and test code:
- B904 (~50): raise-from in except blocks; from None at API/view
  boundaries, from exc where the cause is the direct origin
- B017 (9): pytest.raises(Exception) → specific type or match= arg
- B007 (5): unused loop vars renamed to _
- B027 (1): missing @abstractmethod on DateParserPluginBase.__exit__
- B028 (3): warnings.warn without stacklevel=2 in test utils
- B011 (1): assert False → raise AssertionError()
- B905 (3): zip() without strict=False
- B009 (3): getattr with constant string (auto-fixed)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/documents/tests/test_views.py

@@ -243,7 +243,7 @@ class TestViews(DirectoriesMixin, TestCase):
                     "change": {"users": [], "groups": []},
                 }
             else:
-                assert False, f"Unexpected tag found: {tag['name']}"
+                raise AssertionError(f"Unexpected tag found: {tag['name']}")
 
     def test_list_no_n_plus_1_queries(self) -> None:
         """