Using PostgreSQL is entirely optional and is geared towards more advanced users (the 0.1%). The reason you might want to use PostgreSQL is because it handles huge databases much better than SQLite, which comes bundled with the main autobrr application.
Nevertheless, if you want to use PostgreSQL with autobrr, then add this to your autobrr configuration file and restart your autobrr instance:
# Database config
databaseType = "postgres"
postgresHost = "localhost"
postgresPort = 5432
postgresDatabase = "autobrr"
postgresUser = "autobrr"
postgresPass = "s0meth!ng-l0ng-4nd-s3cure"
postgresSSLMode = "disable"
postgresExtraParams = ""
It is recommended to create a new user and database for autobrr. You might be able to migrate your SQLite database to PostgreSQL, but the authors haven't taken a deep look into it.
SQLite still remains the primary option which is actively being tested against.
It's up to you to make sure your postgres instance is secured and not exposed to the internet.