feat: add incident management and maintenance windows

Maintenance windows suppress alerts during planned downtime while checks
continue running. Incidents provide informational tracking. Supports
targeting all monitors, single monitor, or group (applies to children).

New Maint tab in TUI with create/end/delete. Status page, JSON API, and
Prometheus metrics all reflect maintenance state.

internal/tui/tab_sites.go

@@ -207,10 +207,13 @@ func fmtRetries(site models.Site) string {
 	return s
 }
 
-func fmtStatus(status string, paused bool) string {
+func fmtStatus(status string, paused bool, inMaint bool) string {
 	if paused {
 		return warnStyle.Render("PAUSED")
 	}
+	if inMaint {
+		return maintStyle.Render("MAINT")
+	}
 	switch {
 	case status == "DOWN" || status == "SSL EXP":
 		return dangerStyle.Render(status)
@@ -280,7 +283,7 @@ func (m Model) viewSitesTab() string {
 						strconv.Itoa(i + 1),
 						m.zones.Mark(fmt.Sprintf("site-%d", i), icon+" "+limitStr(site.Name, nameW-2)),
 						"group",
-						fmtStatus(site.Status, site.Paused),
+						fmtStatus(site.Status, site.Paused, m.isMonitorInMaintenance(site.ID)),
 						subtleStyle.Render("—"),
 						subtleStyle.Render("—"),
 						subtleStyle.Render(strings.Repeat("·", sparkWidth)),
@@ -313,7 +316,7 @@ func (m Model) viewSitesTab() string {
 					strconv.Itoa(i + 1),
 					m.zones.Mark(fmt.Sprintf("site-%d", i), name),
 					typeIcon(site.Type, false) + " " + site.Type,
-					fmtStatus(site.Status, site.Paused),
+					fmtStatus(site.Status, site.Paused, m.isMonitorInMaintenance(site.ID)),
 					fmtLatency(site.Latency),
 					fmtUptime(hist.Statuses),
 					spark,
@@ -623,7 +626,15 @@ func (m Model) viewDetailPanel() string {
 		b.WriteString(fmt.Sprintf("  %-16s %s\n", subtleStyle.Render(label), value))
 	}
 
-	row("Status", fmtStatus(site.Status, site.Paused))
+	row("Status", fmtStatus(site.Status, site.Paused, m.isMonitorInMaintenance(site.ID)))
+	if m.isMonitorInMaintenance(site.ID) {
+		for _, mw := range m.maintenanceWindows {
+			if mw.Type == "maintenance" && (mw.MonitorID == 0 || mw.MonitorID == site.ID || mw.MonitorID == site.ParentID) {
+				row("Maintenance", maintStyle.Render(mw.Title))
+				break
+			}
+		}
+	}
 	row("Type", site.Type)
 	if site.URL != "" {
 		row("URL", site.URL)