feat: add /api/v1/language endpoint for Go/Python runtime detection (#16952)

Both backends serve GET /api/v1/language. Frontend calls it once and
caches. By this way, front end can know the backend is go or python and
thus can determine which part of logic to load.

---------

Co-authored-by: Claude <noreply@anthropic.com>

api/apps/restful_apis/system_api.py

@@ -62,6 +62,13 @@ def version():
     return get_json_result(data=get_ragflow_version())
 
 
+@manager.route("/language", methods=["GET"])  # noqa: F821
+def language():
+    """Backend runtime language detection for front-end compatibility."""
+    logging.info("Language endpoint called, returning python")
+    return get_json_result(data={"language": "python"})
+
+
 @manager.route("/system/status", methods=["GET"])  # noqa: F821
 @login_required
 def status():