Fix mail queue and logs commands

- Hardcode paths: /usr/sbin/postqueue, /usr/sbin/postsuper, /usr/bin/journalctl
- Fix postqueue: remove -p flag (conflicts with -j)
- Add stderr logging for queue errors

backend/internal/mail/logs.go

@@ -26,7 +26,7 @@ func GetLogs(hours int, filter string) ([]LogEntry, error) {
 		"--no-pager",
 	}
 
-	cmd := exec.CommandContext(ctx, "journalctl", args...)
+	cmd := exec.CommandContext(ctx, "/usr/bin/journalctl", args...)
 	var out bytes.Buffer
 	cmd.Stdout = &out