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/internal/config/config.go

@@ -95,6 +95,11 @@ const (
 	envNotificationAdminEmail = "BACKEND_NOTIFICATION_ADMIN_EMAIL"
 	envNotificationWorkerInterval = "BACKEND_NOTIFICATION_WORKER_INTERVAL"
 	envNotificationMaxAttempts = "BACKEND_NOTIFICATION_MAX_ATTEMPTS"
+
+	envDevSandboxEmail = "BACKEND_DEV_SANDBOX_EMAIL"
+	envDevSandboxEngineImage = "BACKEND_DEV_SANDBOX_ENGINE_IMAGE"
+	envDevSandboxEngineVersion = "BACKEND_DEV_SANDBOX_ENGINE_VERSION"
+	envDevSandboxPlayerCount = "BACKEND_DEV_SANDBOX_PLAYER_COUNT"
 )
 
 // Default values applied when an environment variable is absent.
@@ -157,6 +162,9 @@ const (
 
 	defaultNotificationWorkerInterval = 5 * time.Second
 	defaultNotificationMaxAttempts = 8
+
+	defaultDevSandboxEngineVersion = "0.1.0"
+	defaultDevSandboxPlayerCount = 20
 )
 
 // Allowed values for the closed-set string options.
@@ -193,12 +201,29 @@ type Config struct {
 	Engine EngineConfig
 	Runtime RuntimeConfig
 	Notification NotificationConfig
+	DevSandbox DevSandboxConfig
 
 	// FreshnessWindow mirrors the gateway freshness window and is used by the
 	// push server to bound the cursor TTL.
 	FreshnessWindow time.Duration
 }
 
+// DevSandboxConfig configures the boot-time bootstrap implemented in
+// `backend/internal/devsandbox`. When Email is empty the bootstrap
+// is a no-op, which is the production posture. When Email is set —
+// from `BACKEND_DEV_SANDBOX_EMAIL` in the `tools/local-dev` stack —
+// the bootstrap idempotently provisions a real user, the configured
+// number of dummy participants, a private "Dev Sandbox" game, the
+// matching memberships, and drives the lifecycle to `running`. The
+// engine image and engine version refer to a row that the bootstrap
+// also seeds in `engine_versions`.
+type DevSandboxConfig struct {
+	Email string
+	EngineImage string
+	EngineVersion string
+	PlayerCount int
+}
+
 // LoggingConfig stores the parameters used by the structured logger.
 type LoggingConfig struct {
 	// Level is the zap level name (e.g. "debug", "info", "warn", "error").
@@ -469,6 +494,10 @@ func DefaultConfig() Config {
 			WorkerInterval: defaultNotificationWorkerInterval,
 			MaxAttempts: defaultNotificationMaxAttempts,
 		},
+		DevSandbox: DevSandboxConfig{
+			EngineVersion: defaultDevSandboxEngineVersion,
+			PlayerCount: defaultDevSandboxPlayerCount,
+		},
 		Runtime: RuntimeConfig{
 			WorkerPoolSize: defaultRuntimeWorkerPoolSize,
 			JobQueueSize: defaultRuntimeJobQueueSize,
@@ -628,6 +657,13 @@ func LoadFromEnv() (Config, error) {
 		return Config{}, err
 	}
 
+	cfg.DevSandbox.Email = strings.TrimSpace(loadString(envDevSandboxEmail, cfg.DevSandbox.Email))
+	cfg.DevSandbox.EngineImage = strings.TrimSpace(loadString(envDevSandboxEngineImage, cfg.DevSandbox.EngineImage))
+	cfg.DevSandbox.EngineVersion = strings.TrimSpace(loadString(envDevSandboxEngineVersion, cfg.DevSandbox.EngineVersion))
+	if cfg.DevSandbox.PlayerCount, err = loadInt(envDevSandboxPlayerCount, cfg.DevSandbox.PlayerCount); err != nil {
+		return Config{}, err
+	}
+
 	if err := cfg.Validate(); err != nil {
 		return Config{}, err
 	}
@@ -823,6 +859,21 @@ func (c Config) Validate() error {
 		}
 	}
 
+	if email := strings.TrimSpace(c.DevSandbox.Email); email != "" {
+		if _, err := netmail.ParseAddress(email); err != nil {
+			return fmt.Errorf("%s must be a valid RFC 5322 address: %w", envDevSandboxEmail, err)
+		}
+		if strings.TrimSpace(c.DevSandbox.EngineImage) == "" {
+			return fmt.Errorf("%s must not be empty when %s is set", envDevSandboxEngineImage, envDevSandboxEmail)
+		}
+		if strings.TrimSpace(c.DevSandbox.EngineVersion) == "" {
+			return fmt.Errorf("%s must not be empty when %s is set", envDevSandboxEngineVersion, envDevSandboxEmail)
+		}
+		if c.DevSandbox.PlayerCount <= 0 {
+			return fmt.Errorf("%s must be positive when %s is set", envDevSandboxPlayerCount, envDevSandboxEmail)
+		}
+	}
+
 	return nil
 }