Add alert archiving

Alerts can be manually archived (POST /api/alerts/{id}/archive) or
unarchived (DELETE /api/alerts/{id}/archive). A background goroutine
auto-archives resolved alerts older than TERDUT_ARCHIVE_AFTER (default 7d).
GET /api/alerts hides archived alerts by default; ?archived=true shows them.

cmd/terdut/main.go

@@ -41,6 +41,8 @@ func main() {
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
 
+	go api.StartArchiver(ctx, database, cfg.ArchiveAfter)
+
 	go func() {
 		log.Printf("terdut-server %s listening on %s", version, cfg.Addr)
 		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {