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>

README.md

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+# App
+
+React 19 + Spring Boot 3.4 web app backed by PostgreSQL 17.
+
+## Quick start
+
+> **Requires a full JDK** (not just JRE) for Maven compilation. `mise install` below handles this.
+> Without mise: `sudo apt install openjdk-21-jdk`.
+
+```bash
+# 1. Pin toolchain — installs Java 21 JDK, Node 22, Maven, lefthook, gitleaks
+mise install
+
+# 2. Install deps and git hooks (or run the four commands manually)
+mise run setup
+
+# 3. Start the database
+docker compose up -d
+```
+
+Open two terminals:
+
+```bash
+# Backend → http://localhost:8080
+mvn -f backend/pom.xml spring-boot:run
+
+# Frontend → http://localhost:5173  (proxies /api → :8080)
+cd frontend && npm run dev
+```
+
+## Verify everything
+
+```bash
+mise run check       # format-check + lint + test (both stacks)
+```
+
+Without mise:
+
+```bash
+mvn -f backend/pom.xml spotless:check -q
+cd frontend && npx biome check src
+mvn -f backend/pom.xml test -q
+cd frontend && npx vitest run
+```
+
+## Environment
+
+`.env.example` documents all env vars. `mise run setup` copies it to `.env` automatically on first run.