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/paperless/settings/custom.py

@@ -334,7 +334,7 @@ def parse_dateparser_languages(languages: str | None) -> list[str]:
     language_list = languages.split("+") if languages else []
     # There is an unfixed issue in zh-Hant and zh-Hans locales in the dateparser lib.
     # See: https://github.com/scrapinghub/dateparser/issues/875
-    for index, language in enumerate(language_list):
+    for _, language in enumerate(language_list):
         if language.startswith("zh-") and "zh" not in language_list:
             logger.warning(
                 f"Chinese locale detected: {language}. dateparser might fail to parse"