Add user management system with JWT authentication

- Database schema: users, permissions, client_access tables
- Auth: JWT tokens with Argon2 password hashing
- API: login, user CRUD, permission management
- Dashboard: login required, admin Users tab
- Auto-creates initial admin user on first run

server/src/api/mod.rs

@@ -1,5 +1,8 @@
 //! REST API endpoints
 
+pub mod auth;
+pub mod users;
+
 use axum::{
     extract::{Path, State, Query},
     Json,