final touches before a stable release

After this PR we should be able to bump the version of this project to a stable 1.0.0 and focus on new projects. Things I'd like to address:

[x] Only allow supported attachment files
[x] Implement ratelimits for POST /invoices
[ ] Document the API and make sure everything makes sense
[x] Return "OK" from /health
[x] Validate IBAN account numbers
[ ] Add the filenames as labels to the generated pdf to the start of each attached pdf
[ ] (?) support multiple attachment with the same filename
[x] Check for redundant dependencies
[x] Return error on failed mail send
[ ] Improve logging and error handling to the level of a stable product
[ ] Write tests for the POST /invoices path
[ ] Make everything (to a reasonable extent) configurable either through a .toml file or through env variable
[ ] Prevent unnecessary cloning of attachment files
[ ] Update README and add build instructions
[ ] Bump version to 1.0.0

Others are also welcome to contribute to this branch either directly or through a PR

Tietokilta / laskugeneraattori