feat: add reboot warning cron module

[koji-assistant]

Summary

• Adds a test-reboot-warnings module with five independent warning cronjobs: 60, 30, 15, 5, and 1 minute.
• Each cronjob broadcasts its own configured message via Takaro sendMessage, so games like Palworld do not need console commands.
• Supports {minutesRemaining}, {playerCount}, and {serverName} placeholders plus disabled/blank-message skip behavior.

Discord source

• User: trevorjd / Trevor
• Channel: Scheduled messages on Palworld
• Timestamp: 2026-07-16T07:21:35.092Z
• Akari message id: 87914d15-1cc1-4ccd-ae36-e74d5337e698
• Quote: “Anyone know of a module that uses the method that serverMessages uses but allows me to trigger specific messages with separate cron jobs?”

Related: #21

Test plan

• npm run build
• node dist/scripts/module-to-json.js modules/reboot-warnings /tmp/reboot-warnings.json
• node --check on helper and cronjob JS files
• Focused integration test blocked: sandbox login currently returns HTTP 503: no available server from scripts/takaro-auth.sh, so Takaro API push/install/cron trigger verification could not complete in this cron run.
• Real game verification pending; Palworld-specific runtime QA/human testing still needed.

[koji-assistant]

CI follow-up on current head 75a5f4b5eb5d5d2217590cd90f4ba2709d2d527b:

• Build and typecheck: ✅ success
• Integration tests: ❌ blocked before module tests during Bootstrap Takaro stack
• Failure is the known shared image/bootstrap issue, not a reboot-warnings test failure:
  takaro-app-api:latest not found: manifest unknown: Requested image not found

Local/static evidence is still the PR body evidence. Sandbox push/install/cron trigger and Palworld/runtime QA remain pending.