## Summary
- When an event stream expires (TTL), `addQueryToEventStream` and
`removeQueryFromEventStream` now return `false` instead of throwing
`EVENT_STREAM_DOES_NOT_EXIST` as an `InternalServerError`
- Frontend checks the mutation return value and triggers the
destroy/recreate cycle, same recovery behavior without the error path
- Removes `EVENT_STREAM_DOES_NOT_EXIST` from exception code, exception
filter, and frontend graceful error check since it's no longer thrown
## Test plan
- [x] Verify that when an event stream TTL expires, the frontend
silently recreates the stream without error noise in logs/Sentry
- [x] Verify that `NOT_AUTHORIZED` errors still throw correctly on both
mutations
- [ ] Verify that the subscription `onEventSubscription` still works
end-to-end with stream creation, query registration, and heartbeat TTL
refresh
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