scaffold: React 19 + Spring Boot 3.4 + PostgreSQL project

Sets up toolchain pinning, format/lint (Biome + Spotless), lefthook
pre-commit hooks, Docker Compose for Postgres, and trivial passing
tests for both stacks.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

AGENTS.md

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+# App
+
+A full-stack web app: React 19 frontend talking to a Spring Boot 3.4 REST API backed by PostgreSQL 17.
+The frontend is a single-page app; it is not server-side rendered. This is not a monorepo — it is
+one project with two build roots (`frontend/` and `backend/`) under a single git repo.
+
+**Stack:** Vite 6 + React 19 (TypeScript, strict) — Biome 1.9 for format + lint — Vitest + Testing Library for frontend tests.
+Spring Boot 3.4 (Java 21) — Maven — Google Java Format via Spotless — JUnit 5 + AssertJ for backend tests.
+PostgreSQL 17 via Docker Compose; Flyway owns all schema changes.
+
+## Rules
+
+- **Never suppress warnings** with `@SuppressWarnings`, `biome-ignore`, or `// noinspection`; fix the root cause.
+- **All HTTP responses are validated with Zod** on the frontend before touching any field; never access raw `response.data` directly.
+- **Flyway owns the schema** — never `ALTER TABLE` manually or let Hibernate generate DDL. Write a migration instead.
+- **Logs via SLF4J only** on the backend (`LOG_LEVEL` env var controls root level); do not use `System.out.println`.
+- **Transaction boundaries live in the service layer**, not controllers or repositories. No `@Transactional` on `@RestController`.
+
+## Running locally
+
+```bash
+docker compose up -d                              # PostgreSQL on :5432
+mvn -f backend/pom.xml spring-boot:run           # backend on :8080
+cd frontend && npm run dev                        # frontend on :5173 (proxies /api → :8080)
+```
+
+Verify everything at once: `mise run check` (requires mise; otherwise run the four commands in .mise.toml manually).