feat: choose a product's category from the ones T5DE has seen

A product can be filed under another IMVU category than its parent's: any category path the hub has seen on a
revision's parent (and each step of it), within the parent's top-level category, so it stays the same kind of
product (the proxy tells clothing, furniture and rooms apart by category). It's an info field like the others:
lockable, in the history, undone by the same rules. Lists filter by the chosen path, and products show it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/api.md

@@ -87,8 +87,9 @@ sha256 (hex) of `contents` as JSON.
 ## Products, catalogs and libraries
 
 The routes and tokens are in docs/products.md. Products are `{id, name, description, rating, latest, creator,
-creatorUsername, parent, image, categories, status, listed, tags, locked, deleted, removed, publishedAt, createdAt,
-updatedAt}`; `parent`, `image` and `categories` come from the latest revision (its IMVU parent's public info).
+creatorUsername, parent, image, categories, category, status, listed, tags, locked, deleted, removed, publishedAt,
+createdAt, updatedAt}`; `parent` and `image` come from the latest revision (its IMVU parent's public info), and
+`categories` too unless a category was chosen for it (`category`).
 `GET /api/v1/products` takes `q`, `creator`, `category` (an IMVU category ID), `tag`, `catalog` and `limit` (1-200,
 default 60).