Implement backend upload/download API enhancements
- Add S3 multipart upload support for files > 100MB
- Add resumable upload API endpoints (init, upload part, complete, abort, status)
- Add HTTP range request support for partial downloads
- Add HEAD request endpoint for artifact metadata
- Add format-specific metadata extraction (deb, rpm, tar.gz, wheel, jar, zip)
- Add format_metadata column to artifacts table
- Add database migration for schema updates
- Add deduplication indicator in upload response
- Set Accept-Ranges header on downloads
- Return Content-Length header on all downloads
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