feat: rewrite user model using diesel

migrations/2025-01-13-115534_add_auth/up.sql

@@ -1,13 +1,13 @@
 create table users (
-    id integer primary key not null,
+    id bigint primary key not null,
     username text unique not null,
     password_hash text not null,
     admin boolean not null
 );
 
 create table sessions (
-    id integer primary key not null,
-    user_id integer not null
+    id bigint primary key not null,
+    user_id bigint not null
         references users (id)
         on delete cascade,
     unique (id, user_id)