Add HarborForge stack to docker-compose

- Add hf_db_init sidecar that ensures the HarborForge database exists on
  every `compose up` (idempotent CREATE DATABASE IF NOT EXISTS), so the
  shared MySQL instance can host both hangmanlab and harborforge schemas
  without touching existing data.
- Wire hf_backend's DATABASE_URL directly from compose env vars and gate
  it on hf_db_init completing successfully.
- Add a mysqladmin-ping healthcheck on mysql so dependents can wait on
  service_healthy.
- Drop dead Vite runtime envs from hf_frontend (build-time only) and
  make wizard CORS_ORIGINS configurable via HF_FRONTEND_HOST.
- Seed .env.example with all variables the stack reads.

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

mysql-init/10-harborforge.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+# Idempotent HarborForge DB bootstrap. Runs on every `compose up` via the
+# hf_db_init sidecar service: waits for mysql to accept connections, then
+# ensures the HarborForge database exists and the shared app user can use it.
+set -e
+
+HF_DB_NAME="${HF_DB_NAME:-harborforge}"
+MYSQL_HOST="${MYSQL_HOST:-mysql}"
+
+echo "hf_db_init: waiting for mysql at ${MYSQL_HOST}..."
+until mysql -h"${MYSQL_HOST}" -uroot -p"${MYSQL_ROOT_PASSWORD}" -e "SELECT 1" >/dev/null 2>&1; do
+  sleep 2
+done
+
+echo "hf_db_init: ensuring database '${HF_DB_NAME}' exists and granting access to '${DB_USER}'"
+mysql -h"${MYSQL_HOST}" -uroot -p"${MYSQL_ROOT_PASSWORD}" <<EOSQL
+CREATE DATABASE IF NOT EXISTS \`${HF_DB_NAME}\` CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+GRANT ALL PRIVILEGES ON \`${HF_DB_NAME}\`.* TO '${DB_USER}'@'%';
+FLUSH PRIVILEGES;
+EOSQL
+
+echo "hf_db_init: done"