fix: resolve all pre-existing ruff lint errors

Fix E501 line-too-long in backlog.py, network.py, test_network.py.
Fix F541 f-string-without-placeholders in network.py.
Fix I001 unsorted imports in network.py.
Remove unused datetime import in test_cert.py (F401).

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

tests/test_cert.py

@@ -91,7 +91,6 @@ class TestGenerateCert:
         assert fp1 != fp2  # New cert = new fingerprint
 
     def test_custom_validity_days(self, data_dir: Path) -> None:
-        import datetime
         from cryptography import x509 as x509_mod
         pem = generate_cert(data_dir, "libera", "testnick", validity_days=365)
         cert_data = pem.read_bytes()