<fix>[ai]: use TIMESTAMP with default 1970 to fix MySQL 5.5 compatibility

Resolves: ZSTAC-72079

Change-Id: Ie345ac5c788d4b53912ee80ec7dd437bf149601f

Author: Shangmin Dou

conf/db/upgrade/V5.5.12__schema.sql

@@ -6,7 +6,7 @@ CALL CREATE_INDEX('GpuDeviceSpecVO', 'idx_gpu_spec_normalized_model', 'normalize
 -- Previously these values were only stored inside the opaque JSON TEXT field,
 -- making them invisible to ZQL ORDER BY queries.
 CALL ADD_COLUMN('ModelEvaluationTaskVO', 'totalScore', 'DOUBLE', 1, NULL);
-CALL ADD_COLUMN('ModelEvaluationTaskVO', 'endTime', 'TIMESTAMP', 1, NULL);
+CALL ADD_COLUMN('ModelEvaluationTaskVO', 'endTime', 'TIMESTAMP', 1, '1970-01-02 00:00:00');
 
 -- Add indexes to support efficient sorting
 CALL CREATE_INDEX('ModelEvaluationTaskVO', 'idx_ModelEvaluationTaskVO_totalScore', 'totalScore');