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/cmd/backend/main.go

@@ -26,7 +26,6 @@ import (
 	"galaxy/backend/internal/app"
 	"galaxy/backend/internal/auth"
 	"galaxy/backend/internal/config"
-	"galaxy/backend/internal/devsandbox"
 	"galaxy/backend/internal/diplomail"
 	"galaxy/backend/internal/diplomail/detector"
 	"galaxy/backend/internal/diplomail/translator"
@@ -274,29 +273,18 @@ func run(ctx context.Context) (err error) {
 	)
 	runtimeGateway.svc = runtimeSvc
 
-	// Run a single reconciliation pass before the dev-sandbox
-	// bootstrap so any runtime row pointing at a vanished engine
-	// container (host reboot wiped /tmp/galaxy-game-state/<uuid>;
-	// `tools/local-dev`'s `prune-broken-engines` target reaped the
-	// husk) is already cascaded through `markRemoved` → lobby
-	// `cancelled` by the time the bootstrap walks the sandbox list.
-	// Without this pre-tick the bootstrap would reuse the
-	// soon-to-be-cancelled game and force the developer into a
-	// second `make up` cycle to land a healthy sandbox. Failures are
+	// Run a single reconciliation pass at startup so any runtime row
+	// pointing at a vanished engine container (a host reboot wiped
+	// /tmp/galaxy-game-state/<uuid>; `tools/local-dev`'s
+	// `prune-broken-engines` target reaped the husk) is cascaded
+	// through `markRemoved` → lobby `cancelled` before the server
+	// starts serving requests. Failures are
 	// non-fatal: the periodic ticker started later catches up, and
 	// the worst case degrades to the legacy two-cycle recovery.
 	if err := runtimeSvc.Reconciler().Tick(ctx); err != nil {
 		logger.Warn("pre-bootstrap reconciler tick failed", zap.Error(err))
 	}
 
-	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,