chore: make app REUSE compliant

[Chartman123]

Is your feature request related to a problem? Please describe. We're currently using old licensing headers in our source code

Describe the solution you'd like We should switch to REUSE compliant headers: https://docs.nextcloud.com/server/30/developer_manual/getting_started/codingguidelines.html#license-headers

Describe alternatives you've considered Keep the way it is now