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/lobby_e2e_test.go

@@ -248,8 +248,8 @@ func TestEndToEndPrivateGameFlow(t *testing.T) {
 	}
 }
 
-// TestDeleteGameCascadesEverything pins the contract the dev-sandbox
-// bootstrap relies on: removing a game wipes every referencing row
+// TestDeleteGameCascadesEverything pins the DeleteGame contract:
+// removing a game wipes every referencing row
 // (memberships, applications, invites, runtime_records,
 // player_mappings) in a single SQL statement. Before this is wired
 // the developer's lobby pile up cancelled tiles between