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.

.gitea/workflows/dev-deploy.yaml

@@ -153,7 +153,7 @@ jobs:
           # Compare the freshly-built `galaxy-engine:dev` SHA against
           # every running `galaxy-game-*` container. The backend
           # reconciler adopts pre-existing labelled engine containers
-          # without checking image drift, so a running sandbox would
+          # without checking image drift, so a running game would
           # otherwise keep serving the previous engine code until the
           # container is recycled by hand. This step makes the recycle
           # automatic but only when it is actually needed:
@@ -168,10 +168,7 @@ jobs:
           #     silent state corruption otherwise), and cascade-delete
           #     the lobby `games` row (the FKs in `00001_init.sql`
           #     drop the matching `runtime_records`, `memberships`,
-          #     `player_mappings`, etc. in the same write). The
-          #     `dev-sandbox` bootstrap on the next backend boot finds
-          #     no live sandbox and provisions a fresh one on the new
-          #     engine image.
+          #     `player_mappings`, etc. in the same write).
           #
           # Backend is stopped first to keep the reconciler from
           # racing the recycle (mid-stream adoption / restart). The