feat: Archived alerts tab with archive/unarchive actions

Add a fourth tab (Alerts | Archived | Schedule | Users).
Archived alerts are fetched lazily on first visit using the
archived=true query param on GET /api/alerts.

Press x from the Alerts list or detail to archive an alert;
the non-archived list refreshes immediately. Press x from the
Archived list or detail to unarchive; the archived list
refreshes. Ack/unack are disabled in the Archived detail view.

New API methods: ArchiveAlert (POST), UnarchiveAlert (DELETE).
ArchivedAt field added to the Alert type.

internal/api/types.go

@@ -16,6 +16,7 @@ type Alert struct {
 	AcknowledgedByID *int64            `json:"acknowledged_by_id"`
 	AcknowledgedBy   string            `json:"acknowledged_by"`
 	AcknowledgedAt   *time.Time        `json:"acknowledged_at"`
+	ArchivedAt       *time.Time        `json:"archived_at,omitempty"`
 }
 
 type AlertStats struct {