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>

backend/cmd/backend/main.go

@@ -24,6 +24,7 @@ import (
 	"galaxy/backend/internal/app"
 	"galaxy/backend/internal/auth"
 	"galaxy/backend/internal/config"
+	"galaxy/backend/internal/devsandbox"
 	"galaxy/backend/internal/dockerclient"
 	"galaxy/backend/internal/engineclient"
 	"galaxy/backend/internal/geo"
@@ -265,6 +266,14 @@ func run(ctx context.Context) (err error) {
 	)
 	runtimeGateway.svc = runtimeSvc
 
+	if err := devsandbox.Bootstrap(ctx, devsandbox.Deps{
+		Users:          userSvc,
+		Lobby:          lobbySvc,
+		EngineVersions: engineVersionSvc,
+	}, cfg.DevSandbox, logger); err != nil {
+		return fmt.Errorf("dev sandbox bootstrap: %w", err)
+	}
+
 	notifStore := notification.NewStore(db)
 	notifSvc := notification.NewService(notification.Deps{
 		Store:    notifStore,