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_credits.py

@@ -3,9 +3,8 @@ Tests for the credit system (credits.py).
 
 Pure SQL operations against real in-memory SQLite — no mocking needed.
 """
-from padelnomics.core import utcnow_iso
-
 import pytest
+from padelnomics.core import utcnow_iso
 from padelnomics.credits import (
     InsufficientCredits,
     add_credits,