feat(ha): wire sops secrets and disable NetworkManager for HA servers

- cluster-config.nix: add corosync_authkey sops binary secret
  (/etc/corosync/authkey, mode 0400) and force-disable NetworkManager
  (common config enables it; HA nodes need stable static IP networking)
- hosts/ha-server-{1,2}/host.nix: add host-token.nix import for
  sops-managed beszel-token; add KEY placeholder for beszel hub pairing
- .sops.yaml: add creation rules for secrets/ha-server-{1,2}.yaml and
  secrets/ha-corosync-authkey (admin-only until sync-host-keys.sh runs)
- secrets/ha-server-{1,2}.yaml, secrets/ha-corosync-authkey: stub files
  so eval passes before real secrets are provisioned

Bootstrap order (post-merge):
  1. bash scripts/secrets/sync-host-keys.sh proxmox-ha-server-1
  2. bash scripts/secrets/sync-host-keys.sh proxmox-ha-server-2
  3. sops updatekeys secrets/common.yaml  (grants HA nodes common secrets)
  4. sops secrets/ha-server-{1,2}.yaml   (set beszel-token values)
  5. On node1: corosync-keygen; sops -e --input-type binary
     /etc/corosync/authkey > secrets/ha-corosync-authkey; git add/commit

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HaH1cSGvhogRP5ExoF6nD8

secrets/ha-corosync-authkey

@@ -0,0 +1 @@
+STUB: run cluster-init.sh to generate, then: sops -e --input-type binary /etc/corosync/authkey > secrets/ha-corosync-authkey