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/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';