Implement S3/B2 file backups
Approximate:
- research how Rust works with s3/b2 8h
- discuss s3/b2 storage and credentials 8h
- push file to s3/b2, put it in "s3cache" directory 4h+1h
- functionality to download missing files from s3 3h
- Create tests 6h
- Documentation 4h
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