feat: add user model and registration endpoint

backend/tests/test_auth.py

@@ -0,0 +1,21 @@
+import pytest
+
+
+@pytest.mark.asyncio
+async def test_register_creates_user(client):
+    response = await client.post(
+        "/auth/register",
+        json={"username": "medium1", "password": "spookyspooky"},
+    )
+    assert response.status_code == 201
+    body = response.json()
+    assert body["username"] == "medium1"
+    assert "id" in body
+    assert "password" not in body
+
+
+@pytest.mark.asyncio
+async def test_register_duplicate_username_rejected(client):
+    await client.post("/auth/register", json={"username": "medium1", "password": "spookyspooky"})
+    response = await client.post("/auth/register", json={"username": "medium1", "password": "anotherpass"})
+    assert response.status_code == 409