Reduce chunk size from 32MB to 256KB for proper TCP backpressure

[twdamhore]

Summary

Fixes high memory usage (2GB+) when serving large ISO files.

Root cause: 32MB chunks are too large for TCP. Hyper buffers received data while waiting to write to socket, causing memory to accumulate.

Fix:

• CHUNK_SIZE: 32MB → 8MB
• CHANNEL_CAPACITY: 2 (~16MB max in flight)
• Simplified comments (removed hardcoded values)

Expected results

• Memory usage significantly reduced
• Proper backpressure from TCP layer to disk reads

Test plan

# Start server with new binary
systemctl restart serabutd

# Monitor memory while serving ISO
watch -n1 'ps -o rss= -p $(pgrep serabutd)'

# Trigger ISO download
curl http://localhost:4123/iso/ubuntu-22.04/ubuntu-22.04.5-live-server-amd64.iso -o /dev/null

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