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>

.env.example

@@ -0,0 +1,5 @@
+# Copy to .env — mise run setup does this automatically
+DATABASE_URL=jdbc:postgresql://localhost:5432/app_dev
+DATABASE_USERNAME=app
+DATABASE_PASSWORD=app
+LOG_LEVEL=INFO