fix: use SQLite-compatible space format in utcnow_iso(), fix credits ordering

utcnow_iso() now produces 'YYYY-MM-DD HH:MM:SS' (space separator) matching
SQLite's datetime('now') so lexicographic comparisons like
'published_at <= datetime(now)' work correctly.

Also add `id DESC` tiebreaker to get_ledger() ORDER BY to preserve
insertion order when multiple credits are added within the same second.

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

web/tests/test_supplier_webhooks.py

@@ -6,12 +6,11 @@ Uses the existing client, db, sign_payload from conftest.
 """
 import json
 from datetime import UTC, datetime
-
-from padelnomics.core import utcnow_iso
 from unittest.mock import AsyncMock, patch
 
 import pytest
 from conftest import sign_payload
+from padelnomics.core import utcnow_iso
 
 WEBHOOK_PATH = "/billing/webhook/paddle"
 SIG_HEADER = "Paddle-Signature"