fix: 쿼리 조건, 중복 방지 추가

Author: lvalentine6

src/main/resources/db/migration/V9__migrate_cheer_story_images.sql

@@ -5,7 +5,11 @@ SELECT c.id,
        c.created_at
 FROM cheer c
 WHERE c.image_key IS NOT NULL
-  AND c.image_key != '';
+  AND c.image_key != ''
+  AND NOT EXISTS (SELECT 1
+                  FROM cheer_image ci
+                  WHERE ci.cheer_id = c.id
+                    AND ci.order_index = 1);
 
 INSERT INTO story_image (story_id, image_key, order_index, created_at)
 SELECT s.id,
@@ -14,7 +18,11 @@ SELECT s.id,
        s.created_at
 FROM story s
 WHERE s.image_key IS NOT NULL
-  AND s.image_key != '';
+  AND s.image_key != ''
+  AND NOT EXISTS (SELECT 1
+                  FROM story_image si
+                  WHERE si.story_id = s.id
+                    AND si.order_index = 1);
 
 ALTER TABLE cheer RENAME COLUMN image_key TO _deprecated_image_key;
 ALTER TABLE story RENAME COLUMN image_key TO _deprecated_image_key;