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/membership_direct.go

@@ -20,9 +20,9 @@ type InsertMembershipDirectInput struct {
 // writes as ApproveApplication: the per-game race-name reservation
 // row plus the membership row, and refreshes the in-memory caches.
 //
-// The method is intended for boot-time provisioning by
-// `backend/internal/devsandbox` and similar trusted callers. It is
-// not exposed through any HTTP handler. The caller must guarantee
+// The method is intended for trusted boot-time provisioning and
+// integration tests; it is not exposed through any HTTP handler. The
+// caller must guarantee
 // game.Status == GameStatusEnrollmentOpen — the function returns
 // ErrConflict otherwise — and that the race-name policy and
 // canonical-key invariants are honoured (the implementation reuses
@@ -30,9 +30,8 @@ type InsertMembershipDirectInput struct {
 // or unsuitable name still fails).
 //
 // Idempotency: if a membership for (GameID, UserID) already exists
-// the function returns the existing row without modifying state.
-// This makes the helper safe to call on every backend boot from
-// devsandbox.Bootstrap.
+// the function returns the existing row without modifying state, so
+// the helper is safe to call repeatedly.
 func (s *Service) InsertMembershipDirect(ctx context.Context, in InsertMembershipDirectInput) (Membership, error) {
 	displayName, err := ValidateDisplayName(in.RaceName)
 	if err != nil {