Fixed Slack webhook retry failures

Slack webhook notifications were silently failing when the initial request hit Slack's rate limit.

The problem

Slack returns 429 Too Many Requests when you send too many webhooks in a short window. Our code wasn't handling this status and simply dropped the notification.

The fix

We now retry with exponential backoff:

1. First retry after 1 second

2. Second retry after 4 seconds

3. Third retry after 16 seconds

If all 3 retries fail, the notification is logged and can be manually resent from the dashboard.

Discord webhooks already had retry logic — this brings Slack to parity.