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>

backend/mvnw

@@ -0,0 +1,30 @@
+#!/bin/sh
+# Minimal Maven Wrapper — downloads Apache Maven on first run and caches it.
+set -e
+
+MAVEN_VERSION="3.9.9"
+MAVEN_CACHE="${HOME}/.m2/wrapper/dists/apache-maven-${MAVEN_VERSION}"
+MAVEN_URL="https://repo.maven.apache.org/maven2/org/apache/maven/apache-maven/${MAVEN_VERSION}/apache-maven-${MAVEN_VERSION}-bin.tar.gz"
+
+if [ ! -f "${MAVEN_CACHE}/bin/mvn" ]; then
+  MAVEN_TMP="${MAVEN_CACHE}.tmp.$$"
+  if mkdir "${MAVEN_CACHE}.lock" 2>/dev/null; then
+    echo "Downloading Apache Maven ${MAVEN_VERSION}..." >&2
+    mkdir -p "${MAVEN_TMP}"
+    if command -v curl > /dev/null 2>&1; then
+      curl -fsSL "${MAVEN_URL}" | tar -xz --strip-components=1 -C "${MAVEN_TMP}"
+    elif command -v wget > /dev/null 2>&1; then
+      wget -qO- "${MAVEN_URL}" | tar -xz --strip-components=1 -C "${MAVEN_TMP}"
+    else
+      rm -rf "${MAVEN_TMP}" "${MAVEN_CACHE}.lock"
+      echo "ERROR: curl or wget is required to download Maven" >&2
+      exit 1
+    fi
+    mv "${MAVEN_TMP}" "${MAVEN_CACHE}"
+    rm -rf "${MAVEN_CACHE}.lock"
+  else
+    while [ -d "${MAVEN_CACHE}.lock" ]; do sleep 0.2; done
+  fi
+fi
+
+exec "${MAVEN_CACHE}/bin/mvn" "$@"