feat(tui): add SLA reporting view

Full-screen SLA report accessible via [s] from detail panel.
Computes uptime%, downtime, outage count, longest outage, MTTR,
and MTBF from state_changes table. Includes daily breakdown with
bar chart, switchable time periods (24h/7d/30d/90d), and
scrollable viewport. LATE/STALE treated as UP for SLA purposes.

internal/monitor/monitor.go

@@ -823,3 +823,11 @@ func (e *Engine) GetStateChanges(siteID int, limit int) []models.StateChange {
 	}
 	return changes
 }
+
+func (e *Engine) GetStateChangesSince(siteID int, since time.Time) []models.StateChange {
+	changes, err := e.db.GetStateChangesSince(siteID, since)
+	if err != nil {
+		return nil
+	}
+	return changes
+}