b524fa5e

By: Michael Lynch <git@mtlynch.io>

Extract readVideoUploadParts so the parser never sees a nil tmpPath

The nilable *UploadPath and its three roles (duplicate-file detection,
error-path cleanup, missing-file validation) now live entirely inside
readVideoUploadParts, which guarantees a valid temp path on success.
parseVideoUploadPostRequest works purely with values.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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