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_content.py

@@ -9,6 +9,8 @@ import importlib
 import json
 import sqlite3
 from datetime import date, datetime
+
+from padelnomics.core import utcnow_iso
 from pathlib import Path
 
 import pytest
@@ -70,7 +72,7 @@ async def _create_published_scenario(slug="test-scenario", city="TestCity", coun
 async def _create_article(slug="test-article", url_path="/test-article",
                            status="published", published_at=None):
     """Insert an article row, return its id."""
-    pub = published_at or datetime.utcnow().strftime("%Y-%m-%d %H:%M:%S")
+    pub = published_at or utcnow_iso()
     return await execute(
         """INSERT INTO articles
            (url_path, slug, title, meta_description, country, region,
@@ -936,8 +938,7 @@ class TestRouteRegistration:
 @pytest.fixture
 async def admin_client(app, db):
     """Test client with admin user (has admin role)."""
-    from datetime import datetime
-    now = datetime.utcnow().isoformat()
+    now = utcnow_iso()
     async with db.execute(
         "INSERT INTO users (email, name, created_at) VALUES (?, ?, ?)",
         ("admin@test.com", "Admin", now),