Handle backpressure when streaming file responses

[unprolix]

Summary

• Check the return value of res.write() when streaming large files
• Wait for the drain event before continuing when socket buffer is full
• Prevents unbounded memory accumulation when serving large files to slow clients

Test plan

• Serve large file to artificially throttled client, verify server memory stays bounded
• Serve same file to multiple slow clients simultaneously, verify no memory spike
• Serve file to fast client, verify no performance regression
• Existing unit tests pass
• Linter passes