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If multer failed to write the temp file (e.g. data-volume permission issue),
the error was propagating unhandled through Express, returning a non-JSON 500
that the client couldn't parse — showing only "Upload failed." with no details.
Changes:
- Import mkdir and pre-create UPLOAD_TEMP_DIR on startup (non-blocking)
- Wrap upload.single('file') in a callback so multer errors return JSON
responses with the actual message instead of a bare HTML 500
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>