_ingest_inbound_files ensured the thread uploads dir (mkdir), enumerated it
(iterdir/is_file) to de-duplicate names, and wrote each downloaded attachment
(write_upload_file_no_symlink) directly on the event loop. Offload the directory
prep and every per-file write via asyncio.to_thread; the genuinely async network
read (file_reader) stays on the loop. Externally observable behavior is unchanged.
Found via `make detect-blocking-io` (HIGH: iterdir on an async path).
Add tests/blocking_io/test_channels_ingest.py anchor, verified red->green under
the strict Blockbuster gate.
Co-authored-by: ly-wang19 <ly-wang19@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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