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_mail/tests/test_preprocessor.py

@@ -184,7 +184,12 @@ class TestMailMessageGpgDecryptor(TestMail):
                 EMAIL_GNUPG_HOME=empty_gpg_home,
             ):
                 message_decryptor = MailMessageDecryptor()
-                self.assertRaises(Exception, message_decryptor.run, encrypted_message)
+                self.assertRaisesRegex(
+                    Exception,
+                    "Decryption failed",
+                    message_decryptor.run,
+                    encrypted_message,
+                )
         finally:
             # Clean up the temporary GPG home used only by this test
             try: