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/src/padelnomics/credits.py

@@ -199,6 +199,6 @@ async def get_ledger(supplier_id: int, limit: int = 50) -> list[dict]:
            FROM credit_ledger cl
            LEFT JOIN lead_forwards lf ON cl.reference_id = lf.id AND cl.event_type = 'lead_unlock'
            WHERE cl.supplier_id = ?
-           ORDER BY cl.created_at DESC LIMIT ?""",
+           ORDER BY cl.created_at DESC, cl.id DESC LIMIT ?""",
         (supplier_id, limit),
     )