spam management for forgejo

Usage

create an API token for your admin account, and write it in an api_token file at the root of the repo
remove model.json if you want to start with no pre-existing model of what is spam or not. Or keep it to use the current classifier. The file gets updated when using the tool: the classifier learns from spam/legit decisions and should get progressively better at identifying spam.
run: cargo run
classify users as spam/not spam. Right now the classification is stored locally in db.json, no concrete action is taken. (Ultimately we will want to lock/delete accounts, etc.)

Forgery reads the following environment variables:

ACTUALLY_BAN_USERS: define it (e.g. to true) to actually lock user accounts, send notification emails and eventually delete user accounts. If not defined (the default), no actual action is taken, spammers are only listed in the database. The variable should be set in production, but probably not for testing.

Environment variables that are relevant when ACTUALLY_BAN_USERS=true:

discuss the current design choices for when locking the account/sending a notification email fails. (Current behavior is to periodically retry, avoid deleting if the account could not be locked, but delete the account after the grace period even if the email could not be sent…)
add backend to store data on garage instead of local files
replate the api_token file with a better mechanism: oauth maybe?
improve error handling