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.

tools/local-dev/docker-compose.yml

@@ -122,10 +122,6 @@ services:
       BACKEND_OTEL_TRACES_EXPORTER: none
       BACKEND_OTEL_METRICS_EXPORTER: none
       BACKEND_AUTH_DEV_FIXED_CODE: ${BACKEND_AUTH_DEV_FIXED_CODE:-}
-      BACKEND_DEV_SANDBOX_EMAIL: ${BACKEND_DEV_SANDBOX_EMAIL:-}
-      BACKEND_DEV_SANDBOX_ENGINE_IMAGE: ${BACKEND_DEV_SANDBOX_ENGINE_IMAGE:-}
-      BACKEND_DEV_SANDBOX_ENGINE_VERSION: ${BACKEND_DEV_SANDBOX_ENGINE_VERSION:-}
-      BACKEND_DEV_SANDBOX_PLAYER_COUNT: ${BACKEND_DEV_SANDBOX_PLAYER_COUNT:-}
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
       # Per-game state directories live under the same absolute path