Alertmanager does not expand ${ENV} in its YAML, so the committed config with
${TELEGRAM_BOT_TOKEN}/${TELEGRAM_CHAT_ID} crash-looped it (line 24: cannot
unmarshal !!str `${TELEG...` into int64) - 11k+ restarts on prod, alerting dead.
- rename alertmanager.yml -> alertmanager.yml.template (keeps ${} placeholders)
- deploy.sh: envsubst the template into the (gitignored) alertmanager.yml from
.env, scoped to the two TELEGRAM vars so the {{ }} Go-template message survives
- gitignore the rendered file (contains the bot token)
- warns if the vars are unset
