WP-00: project skeleton with three-level test harness

Cargo workspace (domain, application, infrastructure, api), axum health endpoint
with SPA fallback, Vue 3 + Tailwind frontend, Vitest, Playwright, Makefile,
Gitea Actions CI and README.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

backend/crates/api/tests/healthz.rs

@@ -0,0 +1,12 @@
+//! WP-00 sample integration test: the health endpoint answers 200 with `{"status":"ok"}`.
+mod common;
+
+use axum::http::StatusCode;
+
+#[tokio::test]
+async fn healthz_returns_ok() {
+    let app = common::test_app().await;
+    let res = common::get(&app, "/healthz", None).await;
+    assert_eq!(res.status, StatusCode::OK);
+    assert_eq!(res.json["status"], "ok");
+}