feat: serve built frontend from backend with SPA fallback

backend/tests/test_frontend_serving.py

@@ -0,0 +1,16 @@
+import pytest
+
+
+@pytest.mark.asyncio
+async def test_unknown_path_serves_spa_index(client):
+    response = await client.get("/some/client/side/route")
+    assert response.status_code == 200
+    assert '<div id="root">' in response.text
+
+
+@pytest.mark.asyncio
+async def test_auth_routes_still_work_alongside_spa_fallback(client):
+    response = await client.post(
+        "/auth/register", json={"username": "frontendcheck", "password": "spookyspooky"}
+    )
+    assert response.status_code == 201