[Performance] Move RecordWriter to Background Worker to Prevent Loop Freezing

[DsThakurRawat]

Description

The RecordWriter.write_request call in src/reflex/runtime/server.py (line 2428) is a synchronous, blocking I/O operation called inside an async def handler.

Rationale

Writing large JSONL lines (containing MBs of image base64) and calling flush() synchronously freezes the asyncio event loop. This blocks all concurrent requests and introduces 50-100ms of jitter into the 100Hz robot control loop.

Tasks

• Refactor RecordWriter to use an internal queue.Queue and a background worker thread.
• Use loop.run_in_executor or a non-blocking queue push in the /act handler.

[mvanhorn]

Opened a PR moving record writes onto a bounded queue drained by a daemon worker thread, so disk latency no longer stalls callers (flush-per-record, so a crash loses at most one). 36 tests pass.