fix(tests): replace datetime.utcnow() with utcnow_iso() in test files

Also fixes test_supplier_webhooks.py fromisoformat() comparisons:
expires (naive, from DB) now compared against datetime.now(UTC).replace(tzinfo=None)
to avoid mixing naive/aware datetimes.

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

web/tests/test_feature_flags.py

@@ -7,7 +7,8 @@ Integration tests exercise full request/response flows via Quart test client.
 """
 
 import sqlite3
-from datetime import datetime
+
+from padelnomics.core import utcnow_iso
 from pathlib import Path
 from unittest.mock import AsyncMock, patch
 
@@ -30,7 +31,7 @@ def mock_csrf_validation():
 @pytest.fixture
 async def admin_client(app, db):
     """Test client with an admin-role user session (module-level, follows test_content.py)."""
-    now = datetime.utcnow().isoformat()
+    now = utcnow_iso()
     async with db.execute(
         "INSERT INTO users (email, name, created_at) VALUES (?, ?, ?)",
         ("flags_admin@test.com", "Flags Admin", now),