feat: add user and session models

migrations/2025-02-23-095541_auth/down.sql

@@ -0,0 +1,2 @@
+drop table sessions;
+drop table users;

migrations/2025-02-23-095541_auth/up.sql

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+create table users (
+    id bigint primary key not null,
+    username text unique not null,
+    password_hash text not null
+);
+
+create table sessions (
+    id bigint primary key not null,
+    user_id bigint not null
+        references users (id)
+        on delete cascade,
+    unique (id, user_id)
+);