fix: invalidate caches after deleting QSOs

After deleting all QSOs, invalidate the stats and user caches so the
QSO page shows updated statistics instead of stale cached data.

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

src/backend/services/lotw.service.js

@@ -656,6 +656,10 @@ export async function deleteQSOs(userId) {
 
   logger.info('Deleted QSOs', { userId, count, deletedChanges });
 
+  // Invalidate caches for this user
+  await invalidateStatsCache(userId);
+  await invalidateUserCache(userId);
+
   return count;
 }