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_credits.py

@@ -3,7 +3,7 @@ Tests for the credit system (credits.py).
 
 Pure SQL operations against real in-memory SQLite — no mocking needed.
 """
-from datetime import datetime
+from padelnomics.core import utcnow_iso
 
 import pytest
 from padelnomics.credits import (
@@ -24,7 +24,7 @@ from padelnomics.credits import (
 @pytest.fixture
 async def supplier(db):
     """Supplier with credit_balance=100, monthly_credits=30, tier=growth."""
-    now = datetime.utcnow().isoformat()
+    now = utcnow_iso()
     async with db.execute(
         """INSERT INTO suppliers
            (name, slug, country_code, region, category, tier,
@@ -41,7 +41,7 @@ async def supplier(db):
 @pytest.fixture
 async def lead(db):
     """Lead request with heat_score=warm, credit_cost=20."""
-    now = datetime.utcnow().isoformat()
+    now = utcnow_iso()
     async with db.execute(
         """INSERT INTO lead_requests
            (lead_type, heat_score, credit_cost, status, created_at)
@@ -154,7 +154,7 @@ class TestAlreadyUnlocked:
         assert await already_unlocked(supplier["id"], lead["id"]) is False
 
     async def test_returns_true_after_unlock(self, db, supplier, lead):
-        now = datetime.utcnow().isoformat()
+        now = utcnow_iso()
         await db.execute(
             """INSERT INTO lead_forwards (lead_id, supplier_id, credit_cost, created_at)
                VALUES (?, ?, 20, ?)""",
@@ -210,7 +210,7 @@ class TestUnlockLead:
 
     async def test_raises_insufficient_credits(self, db, lead):
         """Supplier with only 5 credits tries to unlock a 20-credit lead."""
-        now = datetime.utcnow().isoformat()
+        now = utcnow_iso()
         async with db.execute(
             """INSERT INTO suppliers
                (name, slug, country_code, region, category, tier,
@@ -247,7 +247,7 @@ class TestMonthlyRefill:
 
     async def test_noop_when_no_monthly_credits(self, db):
         """Supplier with monthly_credits=0 gets no refill."""
-        now = datetime.utcnow().isoformat()
+        now = utcnow_iso()
         async with db.execute(
             """INSERT INTO suppliers
                (name, slug, country_code, region, category, tier,