Add webhook notifications for evaluation failures

[irparent]

When an evaluation fails (score below threshold), there's no way to get notified. Teams running Iris in production need alerting.

Goal: Add optional webhook notifications when evaluations fail.

Acceptance criteria:

• New config option webhooks in config.json
• POST to configured URL when any evaluation scores below threshold
• Webhook payload includes: trace_id, eval_type, score, failed rules, timestamp
• Retry logic (3 attempts with exponential backoff)
• Configurable threshold per eval type

Resources:

• Eval engine: src/eval/
• Custom rules guide
• API reference