Add per-market alert cooldown to avoid spam

When a market is very active (e.g. election night), a single threshold can trigger dozens of alerts in minutes. A cooldown setting would help.

**Proposed config:**
```yaml
cooldown_per_market: 300  # seconds — only alert once per market per 5 min
```

Implementation: keep a dict of `{market_id: last_alert_timestamp}` and skip if within cooldown window.