feat(media): 媒体信息补充 TVDB slug 字段，修复 TheTvDb 数字 ID 链接 404

TVDB 已弃用数字 ID 直达 URL，仅 slug 形式可访问。详情接口在电视剧有 TVDB ID 时通过轻量 get_series 接口获取 slug，并随 MediaInfo 合并下发。Closes jxxghp/MoviePilot-Frontend#643

app/chain/media.py

@@ -707,7 +707,7 @@ class MediaChain(ChainBase, ConfigReloadMixin, metaclass=Singleton):
             mediainfo.category = tmdb_media.category
         if not mediainfo.genre_ids:
             mediainfo.genre_ids = list(tmdb_media.genre_ids or [])
-        for field in ("imdb_id", "tvdb_id", "collection_id"):
+        for field in ("imdb_id", "tvdb_id", "tvdb_slug", "collection_id"):
             if not getattr(mediainfo, field, None):
                 setattr(mediainfo, field, getattr(tmdb_media, field, None))
         return mediainfo