Limit number of downloads

[paulgalow]

It might be useful to have an optional feature to limit the amount of downloads.

Potential benefits include

• Limiting the blast radius of denial of wallet attacks
• Enabling "self-destructing" download links that are valid for one download only, e.g. for sending sensitive information

Thoughts on how to implement that

• Use DynamoDB table item attributes to track current and max downloads counters
• Alternative: Store current and max downloads counters as object metadata on each uploaded file. Would alleviate need for separate database but would add more latency since S3 would effectively act as a database. The biggest downside I see for that option: in order to change object metadata you need to create a copy of that object.