feat(billing): record subscription-covered token usage in usage records

Write h5_usage_records when plan quota fully covers a session bill so
WeChat and H5 consumption stays visible in admin usage history.

Co-authored-by: Cursor <cursoragent@cursor.com>

db.mjs

@@ -194,6 +194,12 @@ export async function migrateSchema(pool) {
       `ALTER TABLE h5_usage_records ADD UNIQUE KEY uniq_h5_usage_request_id (request_id)`,
     );
   }
+  if (!(await columnExists(pool, 'h5_usage_records', 'billing_source'))) {
+    await pool.query(
+      `ALTER TABLE h5_usage_records
+       ADD COLUMN billing_source VARCHAR(16) NOT NULL DEFAULT 'wallet' AFTER balance_after_cents`,
+    );
+  }
 
   const assetForeignKeys = [
     {