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/tui/tui.go

@@ -71,6 +71,7 @@ const (
 	stateConfirmDelete
 	stateFormMaint
 	stateHistory
+	stateSLA
 )
 
 type Model struct {
@@ -96,8 +97,16 @@ type Model struct {
 	historyViewport viewport.Model
 	historyChanges  []models.StateChange
 	historySiteName string
-	isAdmin         bool
-	zones           *zone.Manager
+
+	slaViewport       viewport.Model
+	slaReport         monitor.SLAReport
+	slaDailyBreakdown []monitor.DayReport
+	slaSiteName       string
+	slaSiteID         int
+	slaPeriodIdx      int
+
+	isAdmin bool
+	zones   *zone.Manager
 
 	deleteID   int
 	deleteName string