feat: proper push monitor lifecycle — PENDING, LATE, DOWN states

Push monitors no longer lie about status:

- PENDING stays until first heartbeat (no auto-promote to UP)
- LATE state (amber) when overdue but within grace period
- DOWN only after grace period expires
- Grace period = interval/2, minimum 60s

RecordHeartbeat now handles all transitions:
- PENDING → UP (first heartbeat, logged)
- LATE → UP (late arrival, logged)
- DOWN → UP (recovery, alert + state change persisted)

TUI updates:
- LATE rendered in amber/warning color
- Status bar shows LATE count separately
- Tab badge shows ⚠ for late monitors
- Sort order: DOWN > LATE > UP > PENDING > PAUSED
- Detail panel shows error for LATE monitors

Inspired by Healthchecks.io state machine (new/up/grace/down).

internal/tui/tab_sites.go

@@ -302,6 +302,8 @@ func fmtStatus(status string, paused bool, inMaint bool) string {
 	switch status {
 	case "DOWN", "SSL EXP":
 		return dangerStyle.Render(status)
+	case "LATE":
+		return warnStyle.Render(status)
 	case "PENDING":
 		return subtleStyle.Render(status)
 	default:
@@ -412,7 +414,7 @@ func (m Model) viewSitesTab() string {
 					name = limitStr(name, nameW)
 				}
 
-				if (site.Status == "DOWN" || site.Status == "SSL EXP") && site.LastError != "" {
+				if (site.Status == "DOWN" || site.Status == "SSL EXP" || site.Status == "LATE") && site.LastError != "" {
 					nameLen := len([]rune(name))
 					errSpace := nameW - nameLen - 1
 					if errSpace > 10 {
@@ -764,7 +766,7 @@ func (m Model) viewDetailPanel() string {
 
 	row("Status", fmtStatus(site.Status, site.Paused, m.isMonitorInMaintenance(site.ID)))
 
-	if (site.Status == "DOWN" || site.Status == "SSL EXP") && site.LastError != "" {
+	if (site.Status == "DOWN" || site.Status == "SSL EXP" || site.Status == "LATE") && site.LastError != "" {
 		row("Error", dangerStyle.Render(limitStr(site.LastError, 60)))
 	}