Feature: Add free alerting integration (Discord webhook preferred)

[1kuna]

We need hands-off alerting for node/training failures.

Scope:

• Add alert sender with a pluggable backend (Discord webhook preferred for free tier), alternative: Slack webhook or SMTP email (free options).
• Trigger on critical self-check failures (S3 down, missing env, vendor outage), and on repeated job failures.
• Integrate into node loop and training loop; keep alerts rate-limited and deduplicated.

Proposed approach:

• Implement ops/alerts.py with send_alert(title, body) and backends: discord_webhook, slack_webhook, email(smtp).
• Configure via .env: ALERT_BACKEND, DISCORD_WEBHOOK_URL, SLACK_WEBHOOK_URL, SMTP_*.
• Add a small circuit breaker to avoid alert storms.

Acceptance:

• Simulated S3 outage -> one alert created and logged, retries after backoff without spamming.
• Missing vendor keys -> warning notifies once per day until resolved.
• Documentation in README with setup instructions.

Notes:

• Start with Discord Webhook (free) as default backend; others optional.