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/server/server.go

@@ -35,6 +35,7 @@ var statusTpl = template.Must(template.New("status").Parse(`
 		.PENDING { background: #e0af68; color: #1a1b26; }
 		.SSL-EXP { background: #e0af68; color: #1a1b26; }
 		.PAUSED { background: #565f89; color: #c0caf5; }
+		.MAINT { background: #bb9af7; color: #1a1b26; }
 		.summary { display: flex; justify-content: center; gap: 16px; margin-bottom: 24px; font-size: 0.95em; font-weight: 600; }
 		.summary span { padding: 4px 12px; border-radius: 6px; }
 		.summary .s-up { color: #9ece6a; }
@@ -68,15 +69,17 @@ var statusTpl = template.Must(template.New("status").Parse(`
 		}
 
 		function renderSummary(sites) {
-			var up = 0, down = 0, paused = 0, total = sites.length;
+			var up = 0, down = 0, paused = 0, maint = 0, total = sites.length;
 			for (var i = 0; i < sites.length; i++) {
 				if (sites[i].Paused) { paused++; continue; }
+				if (sites[i].Status === 'MAINT') { maint++; continue; }
 				if (sites[i].Status === 'UP') up++;
 				else if (sites[i].Status === 'DOWN') down++;
 			}
 			var el = document.getElementById('summary');
 			var parts = ['<span class="s-total">' + up + '/' + total + ' UP</span>'];
 			if (down > 0) parts.push('<span class="s-down">' + down + ' DOWN</span>');
+			if (maint > 0) parts.push('<span style="color:#bb9af7">' + maint + ' MAINT</span>');
 			if (paused > 0) parts.push('<span class="s-paused">' + paused + ' PAUSED</span>');
 			el.innerHTML = parts.join('<span style="color:#383838">·</span>');
 		}
@@ -110,7 +113,7 @@ var statusTpl = template.Must(template.New("status").Parse(`
 			renderSummary(sites);
 			for (var i = 0; i < sites.length; i++) {
 				var s = sites[i];
-				var st = s.Paused ? 'PAUSED' : s.Status;
+				var st = s.Status === 'MAINT' ? 'MAINT' : s.Paused ? 'PAUSED' : s.Status;
 				var cls = cssClass(st);
 				var meta = esc(s.Type) + ' | ' + (s.Type === 'http' ? esc(s.URL) : 'Heartbeat Monitor');
 				var lc = s.LastCheck ? new Date(s.LastCheck).toLocaleTimeString('en-GB', {hour12: false}) : '—';
@@ -359,8 +362,24 @@ func Start(cfg ServerConfig, s store.Store, eng *monitor.Engine) {
 		mux.HandleFunc("/status", func(w http.ResponseWriter, r *http.Request) { renderStatusPage(w, cfg.Title, eng) })
 		mux.HandleFunc("/status/json", func(w http.ResponseWriter, r *http.Request) {
 			state := eng.GetLiveState()
+			activeWindows, _ := s.GetActiveMaintenanceWindows()
+			maintSet := make(map[int]bool)
+			allInMaint := false
+			for _, mw := range activeWindows {
+				if mw.Type != "maintenance" {
+					continue
+				}
+				if mw.MonitorID == 0 {
+					allInMaint = true
+				} else {
+					maintSet[mw.MonitorID] = true
+				}
+			}
 			for id, site := range state {
 				site.Token = ""
+				if allInMaint || maintSet[site.ID] || (site.ParentID > 0 && maintSet[site.ParentID]) {
+					site.Status = "MAINT"
+				}
 				state[id] = site
 			}
 			w.Header().Set("Content-Type", "application/json")