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/store.go

@@ -236,9 +236,8 @@ func (s *Store) ListMyGames(ctx context.Context, userID uuid.UUID) ([]GameRecord
 // referencing table (memberships / applications / invites /
 // runtime_records / player_mappings — all declared with ON DELETE
 // CASCADE in `00001_init.sql`). Idempotent: returns nil when no row
-// matches. Used by the dev-sandbox bootstrap to scrub terminal
-// games on every backend boot so the developer's lobby never piles
-// up cancelled tiles.
+// matches. A hard delete for trusted callers and integration tests;
+// production lifecycle uses cancel / finish.
 func (s *Store) DeleteGame(ctx context.Context, gameID uuid.UUID) error {
 	g := table.Games
 	stmt := g.DELETE().WHERE(g.GameID.EQ(postgres.UUID(gameID)))