feat(cluster): zero-config horizontal scaling by cloning

An instance decides what it is at startup instead of being told: it
generates its own identity, registers a heartbeat, and campaigns for
each game. Exactly one instance drives a game's rounds and publishes
frames; the rest relay them and forward mutations to the leader. Clone
the VM, boot it, done.

Sessions and the scratch nonce move to Redis. Both were per-instance
state that would have broken behind a load balancer: a token minted by
one clone was unknown to the others, and two clones would have handed
the same nonce to different players, which for the same key means the
same outcome.

Fixes a bug found by running two instances: /api/games read the local
room object, so a follower reported a permanently settled game and its
clients never saw a betting window. Hubs now serve the last frame they
saw, produced or relayed.

Failover measured at 6s after kill -9 on an instance leading two games.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docker-compose.yml

@@ -29,7 +29,11 @@ services:
   arcade:
     build: .
     environment:
+      # On a cloned VM, point these at the core machine instead. They are the
+      # only configuration a clone needs; identity and role are worked out at
+      # runtime. See docs/SCALING.md.
       ARCADE_DSN: postgres://arcade:arcade_dev@postgres:5432/arcade
+      ARCADE_REDIS: redis:6379
       ARCADE_ADDR: ":8080"
       # Development funding. Leave unset in any real deployment.
       ARCADE_DEV_FAUCET: "${ARCADE_DEV_FAUCET:-0}"