feat: notify Discord for new Google signups

[anxi01]

Summary

• send a Discord webhook when a user signs in with Google for the first time
• use a dedicated DISCORD_NEW_USER_WEBHOOK_URL so signup notifications can go to a separate channel from waitlist alerts
• keep signup success independent from Discord delivery failures

Details

• add discord_new_user_webhook_url to server settings
• send the new-user webhook only when google_login creates a user for the first time
• add Discord payload coverage for the new signup embed
• document the extra webhook variable in .env.example and README.md

Testing

• python3 -m pytest tests/test_auth_api.py tests/test_discord_service.py -q
• python3 -m pytest -q