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/lobby/games.go

@@ -274,11 +274,10 @@ func (s *Service) ListFinishedGamesBefore(ctx context.Context, cutoff time.Time)
 // `ON DELETE CASCADE` constraints declared in `00001_init.sql`.
 // Idempotent: returns nil when no game matches.
 //
-// Phase 14 introduces this method for the dev-sandbox bootstrap so a
-// terminal "Dev Sandbox" tile from a previous local-dev session can
-// be scrubbed before a fresh game spawns. Production callers must
-// stay on the regular cancel / finish lifecycle — `DeleteGame` is
-// destructive and bypasses the cascade-notification machinery.
+// `DeleteGame` is destructive — a hard delete that bypasses the
+// cascade-notification machinery — so production callers stay on the
+// regular cancel / finish lifecycle. It is exercised by the lobby
+// integration tests.
 func (s *Service) DeleteGame(ctx context.Context, gameID uuid.UUID) error {
 	if err := s.deps.Store.DeleteGame(ctx, gameID); err != nil {
 		return err