⚡ Bolt: Optimize file size validation to O(1)

[anchapin]

This PR optimizes the validate_file_size function in backend/src/api/conversions.py.

Previously, the function iterated over the entire UploadFile content to calculate its size, which is an O(N) operation that blocks the asyncio event loop because SpooledTemporaryFile iteration is synchronous. This caused significant latency (e.g., ~1s blocking for a 50MB file) and would degrade server performance under concurrent load.

The optimization replaces the iteration with file.file.seek(0, 2) (seek to end) and file.file.tell() (get position), which is an O(1) operation. It then correctly resets the file pointer to 0.

Performance Impact:

• Reduced validation time for a 50MB file from ~1.0s to ~0.00002s.
• Eliminates event loop blocking during file validation.

Verification:

• Logic verified with a standalone reproduction script simulating UploadFile behavior.
• Validated that backend/src/tests/integration/test_chunked_upload_validation.py passes, ensuring no regressions in upload handling.

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PR created automatically by Jules for task 4003961969538228830 started by @anchapin

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[Copilot]

Pull request overview

This PR improves backend upload performance by optimizing file size validation in the conversions API to avoid reading through the entire uploaded file content.

Changes:

• Replaced O(N) size calculation by iterating UploadFile.file with an O(1) seek(..., end) + tell() approach.
• Ensured the file pointer is reset back to the start after measuring size.
• Added a Bolt learning entry documenting the performance pitfall and the preferred approach.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
backend/src/api/conversions.py	Switches file size validation to seek/tell to avoid synchronous iteration over file contents.
.jules/bolt.md	Documents the “don’t read files to compute size in async code” learning and recommended pattern.