fix(media): 缩略图唯一索引改为部分索引，避免软删行占用 episode_id 导致登记冲突

sql/migrate-k114-thumbnail-partial-index.sql

@@ -0,0 +1,7 @@
+-- 缩略图时间轴唯一约束改为部分索引：
+-- media_thumbnail.deleted 为 @TableLogic 逻辑删除，软删行仍占用 episode_id 会导致
+-- ThumbnailRecordService.upsert 的 delete+insert 触发 uk_media_thumbnail_episode 冲突。
+-- 唯一性只约束未删除行（deleted = 0）。
+DROP INDEX IF EXISTS uk_media_thumbnail_episode;
+CREATE UNIQUE INDEX IF NOT EXISTS uk_media_thumbnail_episode
+  ON media_thumbnail (episode_id) WHERE deleted = 0;

sql/schema-phase1.sql

@@ -806,7 +806,7 @@ CREATE TABLE IF NOT EXISTS media_thumbnail (
   deleted        SMALLINT     NOT NULL DEFAULT 0
 );
 CREATE UNIQUE INDEX IF NOT EXISTS uk_media_thumbnail_episode
-  ON media_thumbnail (episode_id);
+  ON media_thumbnail (episode_id) WHERE deleted = 0;
 CREATE INDEX IF NOT EXISTS idx_media_thumbnail_drama
   ON media_thumbnail (drama_id);
 COMMENT ON TABLE media_thumbnail IS '单集缩略图雪碧图与 VTT 时间轴';