`[2026-09-22]` Fleet alert bridge generalized

Fleet alert bridge generalized — beszel-althing → althing-alert-bridge, route registry (/beszel + /kuma), each with its own prefix/footer/parser. /beszel FROZEN and proven byte-identical (its three original tests kept unchanged). An alert that lies about its own source is worse than no alert. Commit 6f0a9b9.