Document every API response body in the OpenAPI spec

kaya-openapi 0.0.8 supports named component schemas, so the routes'
description-only responses become $refs: schemas for the core payloads
(auth providers, lobby, game view, ongoing games, match history,
leaderboard, ratings) live in tavolo.platform.openapi and are registered
through the mixin's components. Engine-defined parts (options echo, game
views, match results) stay open objects on purpose. Error responses now
carry the error envelope schema, and a new suite sweeps the document to
keep every /api response typed.

server/packages/tavolo-platform/README.md

@@ -43,8 +43,8 @@ itself ships no game.
   history: `python -m tavolo.platform.backfill_elo --database-url postgres://…`.
 - `auth.py`, `http.py`, `pagination.py`, `openapi.py`,
   `tortoise_mixin.py` — OIDC helpers, JSON helpers, keyset and
-  page-number pagination, shared OpenAPI fragments, the TortoiseORM
-  lifecycle mixin.
+  page-number pagination, shared OpenAPI fragments and response schemas,
+  the TortoiseORM lifecycle mixin.
 
 ## Adding a game