Better play history handling

Currently, history growth is unbounded by default, and every write is a full rewrite.

`.forever` is the default retention. Combined with #46, a user with a year of listening has a large JSON file that is fully decoded at launch and fully re-encoded on every single play. Nothing caps it, and nothing warns.

Options range from a soft default cap, to append-only storage, to simply not rewriting on every play (batch like the queue snapshot does).

Start by locating the play-history persistence entry point and compare it with the queue snapshot batching path mentioned in the issue. Before coding, determine which retention and write strategy is intended; the issue currently lists several options, so a concrete definition of done still needs agreement.

Written by the indexing model from the issue text.