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_supplier_webhooks.py

@@ -5,7 +5,9 @@ POST real webhook payloads to /billing/webhook/paddle and verify DB state.
 Uses the existing client, db, sign_payload from conftest.
 """
 import json
-from datetime import datetime
+from datetime import UTC, datetime
+
+from padelnomics.core import utcnow_iso
 from unittest.mock import AsyncMock, patch
 
 import pytest
@@ -21,7 +23,7 @@ SIG_HEADER = "Paddle-Signature"
 @pytest.fixture
 async def supplier(db):
     """Supplier with tier=free, credit_balance=0."""
-    now = datetime.utcnow().isoformat()
+    now = utcnow_iso()
     async with db.execute(
         """INSERT INTO suppliers
            (name, slug, country_code, region, category, tier,
@@ -38,7 +40,7 @@ async def supplier(db):
 @pytest.fixture
 async def paddle_products(db):
     """Insert paddle_products rows for all keys the handlers need."""
-    now = datetime.utcnow().isoformat()
+    now = utcnow_iso()
     products = [
         ("credits_25", "pri_credits25", "Credit Pack 25", 999, "one_time"),
         ("credits_100", "pri_credits100", "Credit Pack 100", 3290, "one_time"),
@@ -175,7 +177,7 @@ class TestStickyBoostPurchase:
         assert boosts[0][1] == "active"
         # expires_at should be ~7 days from now
         expires = datetime.fromisoformat(boosts[0][2])
-        assert abs((expires - datetime.utcnow()).days - 7) <= 1
+        assert abs((expires - datetime.now(UTC).replace(tzinfo=None)).days - 7) <= 1
 
         # Verify sticky_until set on supplier
         sup = await db.execute_fetchall(
@@ -202,7 +204,7 @@ class TestStickyBoostPurchase:
         assert len(boosts) == 1
         assert boosts[0][0] == "sticky_month"
         expires = datetime.fromisoformat(boosts[0][1])
-        assert abs((expires - datetime.utcnow()).days - 30) <= 1
+        assert abs((expires - datetime.now(UTC).replace(tzinfo=None)).days - 30) <= 1
 
     async def test_sticky_boost_sets_country(self, client, db, supplier, paddle_products):
         payload = make_transaction_payload(
@@ -387,7 +389,7 @@ class TestBusinessPlanPurchase:
         self, client, db, supplier, paddle_products, test_user,
     ):
         # Need a scenario for the export
-        now = datetime.utcnow().isoformat()
+        now = utcnow_iso()
         async with db.execute(
             """INSERT INTO scenarios (user_id, name, state_json, created_at)
                VALUES (?, 'Test Scenario', '{}', ?)""",