Chore: enable flake8-bugbear (B) default-subset ruff rules

3 B009 (getattr with a constant string, rewrite as attribute access)
hits autofixed. 7 B017 (assert blind Exception) hits: one narrowed
to the actual ValueError raised by bulk_edit.edit_pdf, the other six
suppressed with noqa since the code under test genuinely raises (or
a mock genuinely injects) a bare Exception, so a narrower assertion
would be wrong.

Only the 29 B codes ruff 0.16 enables by default; the rest of
flake8-bugbear needs a separate, deliberate decision.

src/paperless_mail/tests/test_preprocessor.py

@@ -184,7 +184,7 @@ class TestMailMessageGpgDecryptor(TestMail):
                 EMAIL_GNUPG_HOME=empty_gpg_home,
             ):
                 message_decryptor = MailMessageDecryptor()
-                self.assertRaises(Exception, message_decryptor.run, encrypted_message)
+                self.assertRaises(Exception, message_decryptor.run, encrypted_message)  # noqa: B017 - raises a bare Exception on decryption failure
         finally:
             # Clean up the temporary GPG home used only by this test
             try: