Add scripts/check-env.sh for pre-flight env var validation #2

Add a small scripts/check-env.sh that verifies the required env vars (from .env.example) are set before starting the dev server, with clear error messages for each missing var.

Requirements:

• bash, no external dependencies
• read .env.example, extract var names, check each is set in the environment
• exit 1 with a helpful message per missing var, exit 0 if all are set
• keep it simple — this is a pre-flight check, not a validator