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/Makefile

@@ -1,14 +1,17 @@
-.PHONY: help up down logs status rebuild clean psql logs-backend logs-gateway logs-mail wait
+.PHONY: help up down logs status rebuild clean psql logs-backend logs-gateway logs-mail build-engine wait
 
 .DEFAULT_GOAL := help
 
 COMPOSE := docker compose
+REPO_ROOT := $(realpath $(CURDIR)/../..)
+ENGINE_IMAGE := galaxy-engine:local-dev
 
 help:
 	@echo "Local development stack for the Galaxy UI:"
 	@echo "  make up             Build (if needed) and bring up the stack, wait until healthy"
 	@echo "  make down           Stop containers, keep volumes"
 	@echo "  make rebuild        Force rebuild of backend / gateway images and bring up"
+	@echo "  make build-engine   Build the engine image $(ENGINE_IMAGE) used by the dev sandbox"
 	@echo "  make clean          Stop and wipe volumes (postgres data, game state)"
 	@echo "  make logs           Tail all logs"
 	@echo "  make logs-backend   Tail only the backend logs"
@@ -20,14 +23,25 @@ help:
 	@echo "After 'make up', point the UI at the stack with:"
 	@echo "  pnpm -C ui/frontend dev"
 	@echo "and open http://localhost:5173 (UI) plus http://localhost:8025 (Mailpit)."
+	@echo ""
+	@echo "Default login for the auto-provisioned dev sandbox: dev@local.test"
+	@echo "(see BACKEND_DEV_SANDBOX_EMAIL in .env). Login code: 123456."
 
-up:
+up: build-engine
 	$(COMPOSE) up -d --wait
 
-rebuild:
+rebuild: build-engine
 	$(COMPOSE) build --no-cache backend gateway
 	$(COMPOSE) up -d --wait
 
+build-engine:
+	@if docker image inspect $(ENGINE_IMAGE) >/dev/null 2>&1; then \
+		echo "$(ENGINE_IMAGE) already built; skipping (use 'docker rmi $(ENGINE_IMAGE)' to force a rebuild)."; \
+	else \
+		echo "building $(ENGINE_IMAGE)…"; \
+		docker build -t $(ENGINE_IMAGE) -f $(REPO_ROOT)/game/Dockerfile $(REPO_ROOT); \
+	fi
+
 down:
 	$(COMPOSE) down