Fixed temp files leak (T415717)

[AdeelTahir-SE]

Implemented temp file handling improvements for upload flow, including leak prevention and cleanup on both success and failure.

What changed:

1. Added safe temp-file cleanup utility in utils.py
2. Returns structured success/failure result instead of silent behavior.
3. Fixed file handle leaks in utils.py
4. Added guaranteed async cleanup in tasks.py
5. Temp file cleanup now runs in finally block in tasks.py
6. Hardened sync upload validation and cleanup in app.py
7. Validates temp file exists and is non-empty before upload in app.py
8. Cleans temp file in synchronous path via finally, including failure cases in app.py