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/domain/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "domain"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[dependencies]
+async-trait.workspace = true
+chrono.workspace = true
+serde.workspace = true
+thiserror.workspace = true
+uuid.workspace = true
+
+[lints]
+workspace = true

backend/crates/domain/src/lib.rs

@@ -0,0 +1 @@
+//! domain layer