include BeatmapPack id in API response

[notbakaneko]

Doesn't make sense to exclude it anymore, since the id can be used for lookups and the response cursor uses the id instead of the tag

[nanaya]

Last I checked the id shouldn't be used anymore and lookup on id probably doesn't work. If anything the model should pretend the tag being primary key and sort on it instead?

(or maybe there's a different reason primary key is still pack_id but I don't think it warrants adding it to the response. cursors are supposed to be opaque anyway)

[nanaya]

Yeah, tag should always be used when interfacing externally.