feat: MAINT-001 - Fix Ruff violations across all Python source

tests/test_settlement_loop.py

@@ -5,9 +5,7 @@ before the transaction is sent, unconfirmed batches resent by settlement id
 (never double-paying), banned wallets skipped, history queryable over HTTP.
 """
 
-import json
 import time
-import urllib.request
 
 import pytest
 
@@ -68,7 +66,7 @@ def test_threshold_triggers_payout_and_zeroes_pending():
     ledger.charge_request("client", MODEL, 1000, [("wallet-a", 12)])  # 0.018 pending
     treasury = _FakePayoutTreasury()
     tracker = _make_tracker(ledger, treasury, threshold=0.01)
-    port = tracker.start()
+    tracker.start()
     try:
         assert _wait_for(lambda: treasury.batches)
         assert treasury.batches[0] == [("wallet-a", pytest.approx(0.018))]