fix: seven quick-win bug fixes across engine, server, TUI, CLI

1. Alertless monitors no longer spam error logs — triggerAlert
   returns early when alertID <= 0.

2. HTTP response body drained before close — enables connection
   reuse via keep-alive instead of fresh TCP+TLS per check.

3. /api/backup/export enforces GET — was the only endpoint
   accepting any HTTP method.

4. limitStr guards against max < 3 — prevents negative slice
   index panic on very narrow terminals.

5. Filter input accepts multibyte characters — len(msg.Runes)
   instead of len(msg.String()) for proper Unicode support.

6. Startup warning corrected — with no UPTOP_CLUSTER_SECRET,
   endpoints reject (401), not accept. Warning now says so.

7. UPTOP_KEYS file open failure logged — was silently swallowed,
   leaving operators with no admin seeded and no message.

internal/monitor/monitor.go

@@ -864,6 +864,9 @@ func (e *Engine) handleStatusChange(snap models.Site, rawStatus string, code int
 }
 
 func (e *Engine) triggerAlert(alertID int, title, message string) {
+	if alertID <= 0 {
+		return
+	}
 	cfg, err := e.db.GetAlert(context.Background(), alertID)
 	if err != nil {
 		e.AddLog(fmt.Sprintf("Failed to load alert config %d: %v", alertID, err))