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>

docker-compose.yml

@@ -0,0 +1,14 @@
+services:
+  db:
+    image: postgres:17-alpine
+    ports:
+      - "5432:5432"
+    environment:
+      POSTGRES_DB: app_dev
+      POSTGRES_USER: app
+      POSTGRES_PASSWORD: app
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+
+volumes:
+  postgres_data: