fix: use existing MySQL volume from old deployment

Reference the old new-api_mysql_data volume as external so all
existing data is preserved without re-import.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker-compose.yml

@@ -47,3 +47,5 @@ services:
 
 volumes:
   mysql_data:
+    external: true
+    name: new-api_mysql_data