Start Canasta Game

Any participant starts a Canasta game in the chatroom.

Idempotent: if an active game already exists, return it as-is. This prevents race conditions where concurrent /start clicks would each end the previous game and create a new one, effectively killing whoever clicked first.

The body is optional in both senses — it may be absent, and every field in it may be. The Android app ships a bundled build that POSTs this with no body at all, so "no body" has to keep meaning exactly what it meant: the room's language and the Word of the Day pool.