local-dev: boot-time dev sandbox provisions a runnable game on up

Adds backend/internal/devsandbox: an idempotent boot-time hook that,
when BACKEND_DEV_SANDBOX_EMAIL is set, ensures (1) the configured
engine_version row, (2) the real dev user, (3) PlayerCount-1
deterministic dummy users, (4) a private "Dev Sandbox" game with a
year-out turn schedule, (5) memberships for every participant via
the new lobby.Service.InsertMembershipDirect helper, (6) a drive of
the lifecycle to running. Re-running on a populated DB is a no-op;
partial states from earlier crashes are recovered.

tools/local-dev gains the matching env vars in .env, surfaces them
in compose, and acquires a `make build-engine` target that builds
galaxy-engine:local-dev from game/Dockerfile (a prerequisite of
`up`/`rebuild`). The compose game-state mount is changed from a
named volume to a host bind on /tmp/galaxy-game-state so backend's
bind-mount source for spawned engine containers resolves on the
docker daemon.

After `make -C tools/local-dev up`, login as dev@local.test with
the dev code 123456 and the Dev Sandbox already shows up in My
Games. Per-user behaviour for the same email survives a backend
restart.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tools/local-dev/docker-compose.yml

@@ -102,7 +102,7 @@ services:
       BACKEND_SMTP_FROM: "galaxy-backend@galaxy.local"
       BACKEND_SMTP_TLS_MODE: none
       BACKEND_DOCKER_NETWORK: galaxy-local-dev-net
-      BACKEND_GAME_STATE_ROOT: /var/lib/galaxy/game-state
+      BACKEND_GAME_STATE_ROOT: /tmp/galaxy-game-state
       BACKEND_GEOIP_DB_PATH: /var/lib/galaxy/geoip.mmdb
       BACKEND_NOTIFICATION_ADMIN_EMAIL: admin@galaxy.local
       BACKEND_AUTH_CHALLENGE_THROTTLE_MAX: "100"
@@ -111,9 +111,22 @@ services:
       BACKEND_OTEL_TRACES_EXPORTER: none
       BACKEND_OTEL_METRICS_EXPORTER: none
       BACKEND_AUTH_DEV_FIXED_CODE: ${BACKEND_AUTH_DEV_FIXED_CODE:-}
+      BACKEND_DEV_SANDBOX_EMAIL: ${BACKEND_DEV_SANDBOX_EMAIL:-}
+      BACKEND_DEV_SANDBOX_ENGINE_IMAGE: ${BACKEND_DEV_SANDBOX_ENGINE_IMAGE:-}
+      BACKEND_DEV_SANDBOX_ENGINE_VERSION: ${BACKEND_DEV_SANDBOX_ENGINE_VERSION:-}
+      BACKEND_DEV_SANDBOX_PLAYER_COUNT: ${BACKEND_DEV_SANDBOX_PLAYER_COUNT:-}
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
-      - game-state:/var/lib/galaxy/game-state
+      # Per-game state directories live under the same absolute path
+      # both inside the backend container and on the Docker daemon
+      # host (colima VM), so the bind-mount source the backend hands
+      # to the daemon resolves correctly when spawning engine
+      # containers. See backend/internal/runtime/service.go:454.
+      - type: bind
+        source: /tmp/galaxy-game-state
+        target: /tmp/galaxy-game-state
+        bind:
+          create_host_path: true
       - ../../pkg/geoip/test-data/test-data/GeoIP2-Country-Test.mmdb:/var/lib/galaxy/geoip.mmdb:ro
     networks:
       - galaxy-net
@@ -182,5 +195,3 @@ networks:
 volumes:
   postgres-data:
     name: galaxy-local-dev-postgres-data
-  game-state:
-    name: galaxy-local-dev-game-state