opaque: Lower the compute cost

This reduces the time to validate a password, at a minor cost of
security (makes bruteforcing the DB easier, but still really hard).

Note: this invalidates all current passwords. It is recommended to
delete the admin user directly in sqlite:

$ sqlite3 users.db
> DELETE from users WHERE user_id = "admin";

It will be recreated with the reset password when restarting the server.
The admin can then reset other users' passwords.

auth/src/opaque.rs

@@ -27,7 +27,7 @@ impl ArgonHasher {
         mem_cost: 50 * 1024, // 50 MB, in KB
         secret: &[],
         thread_mode: argon2::ThreadMode::Sequential,
-        time_cost: 5,
+        time_cost: 1,
         variant: argon2::Variant::Argon2id,
         version: argon2::Version::Version13,
     };