Fix 1 (bootstrap.py): wrap store.flush() in asyncio.to_thread() inside the
periodic _flush_loop() to avoid blocking the async event loop every 60s.
Synchronous signatures of _start/_stop_model_usage_persistence() and the
one-time seed/shutdown flushes are left unchanged per review scope.
Fix 2 (test_stream_chat_usage_capture.py): add the two-line stream_options
assertion to test_qwen_vl_stream_chat_returns_usage_from_trailing_chunk,
matching the identical check already present in the DeepSeek and Qwen tests.
Fix 3 (design doc): note the single-worker assumption in A3's write strategy
section — multi-worker deployments get last-writer-wins per-row semantics.
Tests: 69 passed, 0 failed (python -m pytest backend/tests -q)
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
