feat(library): descriptive metadata on every entry — platform, year, genres, and friends

Emulation-and-beyond libraries need more than a title and a poster. Every
library shape (GameEntry, CustomEntry, CustomInput, ProviderEntryInput)
now carries a shared, flattened GameMeta: platform, description,
developer, publisher, release_year, genres, tags, region, players. All
fields are optional and flat on the wire, so existing library.json files,
provider plugins, and clients keep working unchanged.

- Installed-store scanners (Steam, Lutris, Heroic, Epic, GOG, Xbox) stamp
  platform=PC; custom/provider entries carry whatever was authored.
- GET /library grows a ?platform= filter (case-insensitive) beside
  ?provider=.
- Console: the add/edit form gets a Details section (round-tripping every
  field, since update replaces the entry), the poster tile a platform
  badge (non-PC only — the store badge already implies PC) and the year.
- plugin-kit: ProviderEntry accepts the same fields (new GameMeta schema,
  spread flat); SDK + OpenAPI spec regenerated.
- pf-client-core decodes platform for future client badges.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

clients/linux/src/cli.rs

@@ -620,6 +620,7 @@ fn mock_library() -> (
         store: store.to_string(),
         title: title.to_string(),
         art: crate::library::Artwork::default(),
+        platform: None,
     };
     let games = vec![
         game("steam:570", "steam", "Dota 2"),