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/models/models.go

@@ -67,8 +67,21 @@ type ProbeNode struct {
 	Version  string
 }
 
-type Backup struct {
-	Sites  []Site        `json:"sites"`
-	Alerts []AlertConfig `json:"alerts"`
-	Users  []User        `json:"users"`
+type MaintenanceWindow struct {
+	ID          int
+	MonitorID   int
+	Title       string
+	Description string
+	Type        string // "maintenance" or "incident"
+	StartTime   time.Time
+	EndTime     time.Time // zero = ongoing
+	CreatedBy   string
+	CreatedAt   time.Time
+}
+
+type Backup struct {
+	Sites              []Site              `json:"sites"`
+	Alerts             []AlertConfig       `json:"alerts"`
+	Users              []User              `json:"users"`
+	MaintenanceWindows []MaintenanceWindow `json:"maintenance_windows,omitempty"`
 }