Update argon2 to 0.6

The only API change affecting us is that PasswordHash moved to
password_hash::phc; use the argon2 crate's top-level re-export instead.
Existing argon2id hashes in password files still verify.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Kmrgkpp9YBJ6AMaWYPPZuD

server/src/repository.rs

@@ -1,9 +1,6 @@
 use common::request::Request;
 
-use argon2::{
-    Argon2,
-    password_hash::{PasswordHash, PasswordVerifier},
-};
+use argon2::{Argon2, PasswordHash, PasswordVerifier};
 
 use std::fs::File;
 use std::io::{BufRead, BufReader};