feat: add user and session entities

src/migrations/004_auth.sql

@@ -0,0 +1,14 @@
+create table users (
+    id integer primary key,
+    username text unique not null,
+    password_hash text not null,
+    admin boolean not null
+);
+
+create table sessions (
+    id integer primary key,
+    user_id integer not null
+        references users (id)
+        on delete cascade,
+    unique (id, user_id)
+);