refactor(models): typed Status constants with IsBroken() predicate

Replace ~150 bare status string comparisons with typed models.Status
constants (StatusUp, StatusDown, StatusPending, StatusLate, StatusStale,
StatusSSLExp). Single IsBroken() method replaces the duplicated
isBroken lambda in monitor.go and isDown function in sla.go.

Adding a new status value (e.g. DEGRADED) now requires one constant
definition instead of grep-and-pray across 16 files.

CheckResult.Status stays string — the checker is the boundary between
raw protocol results and typed status. Cast happens at the edge in
handleStatusChange.

internal/tui/update.go

@@ -455,7 +455,7 @@ func (m *Model) handleSLAData(msg slaDataMsg) (tea.Model, tea.Cmd) {
 	}
 	period := slaPeriods[msg.periodIdx]
 
-	var currentStatus string
+	var currentStatus models.Status
 	for _, s := range m.sites {
 		if s.ID == msg.siteID {
 			currentStatus = s.Status