Auto-annotate worker blocks the event loop on CPU inference

Follow-up from #162 (detection-review chunk 1).

Observation

auto_annotate_sequence is an async procrastinate task, but its body runs synchronous, blocking work directly in the coroutine:

• bucket.download_file(...) (sync boto3 S3 download)
• detector.predict(...) (CPU-bound ONNX inference)

These block the worker's event loop for the duration of each detection.

Impact

Harmless today: procrastinate's default concurrency is 1, so jobs run one at a time anyway and there is nothing else on the loop to starve. It only becomes a problem if worker concurrency is raised (--concurrency > 1) or multiple async tasks are expected to progress in parallel — then everything serializes behind the blocking inference.

Suggested fix (when/if concurrency is raised)

Offload the blocking calls to a thread executor, e.g. await asyncio.to_thread(detector.predict, img) and await asyncio.to_thread(bucket.download_file, key), so the loop stays responsive.

Notes

• No action needed while concurrency stays at 1; filing so it is not forgotten if the worker is scaled.

Start by locating the auto_annotate_sequence procrastinate task and inspect its bucket.download_file and detector.predict calls. Confirm how worker concurrency is configured, then verify that CPU inference and S3 downloads no longer block other async work when concurrency exceeds 1.