Add chunked general download uploads

[mefree2098]

Summary

• add resumable chunk upload/status support for admin-managed general downloads
• keep existing one-shot upload endpoint while allowing large ScoreFlow seed uploads to complete in smaller requests
• cover chunk resume/finalize/hash behavior in storage tests

Verification

• NODE_ENV=test node --test tests/generalDownloadStorage.test.js
• node --check server/services/generalDownloadStorage.js && node --check server/routes/generalDownloadRoutes.js
• npm test (612/612)

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Isolate chunk uploads per session

Using a single deterministic staging path for every upload to the same target allows concurrent sessions to write into the same temp file. In writeDownloadChunk, two clients uploading the same path (or one retrying from offset=0 while another is in progress) will share tempPath, and the later request can truncate/overwrite bytes from the other session, producing a corrupted finalized file even though both chunk requests individually return success.

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