refactor(dev): remove the dev-sandbox bootstrap everywhere

Stage 1 of the dev-as-prod-mirror rework. The auto-provisioned "Dev
Sandbox" game and dummy users are removed so the dev contour starts
empty like prod; the separate legacy-report loader stays as the
test-data path.

- delete backend/internal/devsandbox (package + tests)
- drop the bootstrap call + DevSandboxConfig (struct, Config field,
  BACKEND_DEV_SANDBOX_* env, defaults, loader, validation)
- strip BACKEND_DEV_SANDBOX_* from dev-deploy + local-dev compose and
  .env.example; the generic engine-recycle / prune-broken-engines logic
  stays (it serves real games)
- update tooling docs (dev-deploy README + KNOWN-ISSUES, local-dev
  README + Makefile) and stale comments; DeleteGame and
  InsertMembershipDirect remain (exercised by lobby integration tests)

No app behaviour change beyond not auto-creating the sandbox game.

backend/internal/config/config.go

@@ -105,11 +105,6 @@ const (
 	envDiplomailTranslatorTimeout = "BACKEND_DIPLOMAIL_TRANSLATOR_TIMEOUT"
 	envDiplomailTranslatorMaxAttempts = "BACKEND_DIPLOMAIL_TRANSLATOR_MAX_ATTEMPTS"
 	envDiplomailWorkerInterval = "BACKEND_DIPLOMAIL_WORKER_INTERVAL"
-
-	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.
@@ -178,9 +173,6 @@ const (
 	defaultDiplomailTranslatorTimeout = 10 * time.Second
 	defaultDiplomailTranslatorMaxAttempts = 5
 	defaultDiplomailWorkerInterval = 2 * time.Second
-
-	defaultDevSandboxEngineVersion = "0.1.0"
-	defaultDevSandboxPlayerCount = 20
 )
 
 // Allowed values for the closed-set string options.
@@ -219,29 +211,12 @@ type Config struct {
 	Runtime RuntimeConfig
 	Notification NotificationConfig
 	Diplomail DiplomailConfig
-	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").
@@ -572,10 +547,6 @@ func DefaultConfig() Config {
 			TranslatorMaxAttempts: defaultDiplomailTranslatorMaxAttempts,
 			WorkerInterval: defaultDiplomailWorkerInterval,
 		},
-		DevSandbox: DevSandboxConfig{
-			EngineVersion: defaultDevSandboxEngineVersion,
-			PlayerCount: defaultDevSandboxPlayerCount,
-		},
 		Runtime: RuntimeConfig{
 			WorkerPoolSize: defaultRuntimeWorkerPoolSize,
 			JobQueueSize: defaultRuntimeJobQueueSize,
@@ -755,13 +726,6 @@ 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
 	}
@@ -973,21 +937,6 @@ 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
 }