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/.env.example

@@ -7,12 +7,6 @@
 # baked into `docker-compose.yml`, so this file documents the knobs
 # rather than driving them.
 
-# Auto-provisioned sandbox bootstrap. Empty disables the bootstrap.
-BACKEND_DEV_SANDBOX_EMAIL=dev@galaxy.lan
-BACKEND_DEV_SANDBOX_ENGINE_IMAGE=galaxy-engine:dev
-BACKEND_DEV_SANDBOX_ENGINE_VERSION=0.1.0
-BACKEND_DEV_SANDBOX_PLAYER_COUNT=20
-
 # `123456` short-circuits the email-code path for the dev account.
 # This is also the docker-compose default — set the variable to an
 # empty string here when the environment must rely on real Mailpit