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/dev-deploy/docker-compose.yml

@@ -127,15 +127,6 @@ services:
       # bcrypt-hashed code is single-use). Set the var to an empty
       # string in `.env` to disable.
       BACKEND_AUTH_DEV_FIXED_CODE: ${BACKEND_AUTH_DEV_FIXED_CODE:-123456}
-      # Long-lived dev environment always bootstraps the "Dev Sandbox"
-      # game owned by this email so a freshly redeployed stack already
-      # has one ready-to-play game in the lobby. Set the variable to an
-      # empty string in `.env` to disable the bootstrap (e.g. for a
-      # cold-start QA pass).
-      BACKEND_DEV_SANDBOX_EMAIL: ${BACKEND_DEV_SANDBOX_EMAIL:-dev@galaxy.lan}
-      BACKEND_DEV_SANDBOX_ENGINE_IMAGE: ${BACKEND_DEV_SANDBOX_ENGINE_IMAGE:-galaxy-engine:dev}
-      BACKEND_DEV_SANDBOX_ENGINE_VERSION: ${BACKEND_DEV_SANDBOX_ENGINE_VERSION:-0.1.0}
-      BACKEND_DEV_SANDBOX_PLAYER_COUNT: ${BACKEND_DEV_SANDBOX_PLAYER_COUNT:-20}
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock
       # Per-game state directories live under the same absolute path