feat(api-watermark): 水印接口优化

packages/database/migrations/007_media_upload_cleanup.sql

@@ -0,0 +1,9 @@
+-- 上传意图到达终态后，Worker 需要可靠记录临时对象是否已从对象存储清理。
+SET search_path TO atlas, public;
+
+ALTER TABLE media_upload_intents
+    ADD COLUMN object_cleaned_at TIMESTAMPTZ;
+
+CREATE INDEX ix_media_upload_intents_cleanup
+    ON media_upload_intents (workspace_id, status, created_at)
+    WHERE status IN ('failed', 'expired') AND object_cleaned_at IS NULL;