feat(clan): add vps-relay + strip bbbot cloudflared 🚢

Stage 4.5: declare a Hetzner-hosted reverse-proxy VPS as a clan machine.

- nixos/hosts/vps-relay.nix: Debian→NixOS cx23 in hel1. Caddy at public
  80/443 reverse-proxies navidrome.dannydannydanny.me and
  bbbot.dannydannydanny.me over ZT to sunken-ship.
- nixos/disko-cloud.nix: simple GPT + ext4 root, no LUKS — cloud provider
  has physical disk anyway and there's no operator at boot.
- flake-modules/clan.nix: register vps-relay as an inventory machine,
  zerotier peer, internet networking target at its clan-generated ZT
  IPv6, and add vps-relay.clan to clanHostsModule /etc/hosts.
- sunken-ship fitness-bot: drop pkgs.cloudflared from PATH + set
  WEBAPP_URL=https://bbbot.dannydannydanny.me. Paired with the bbbot
  upstream patch (start.py honors env WEBAPP_URL and skips cloudflared
  when set) — once the 15-min fitness-bot-pull timer pulls that change,
  bbbot will stop churning trycloudflare.com URLs.

Vars (zerotier identity/ip + sops machine key) generated on sunken-ship
because clan's hermetic sandbox on macOS fails to run the zerotier
identity generator (same workaround as for data-mesher earlier).

VPS install flow: Hetzner-created Debian box, then `clan machines
install vps-relay --target-host root@<public-ipv4>` reinstalls to
NixOS; subsequent updates go over ZT.

nixos/hosts/sunken-ship.nix

@@ -120,6 +120,11 @@
   # Code: https://github.com/DannyDannyDanny/bigbiggerbiggestbot cloned at /home/danny/tg_fitness_bot
   # Bot token: ~danny/.secrets/bigbiggerbiggestbot
   # Deployment: fitness-bot-pull timer below runs every 15 min, git pulls, restarts service on changes.
+  #
+  # Mini App URL is fronted by Caddy on the vps-relay host at
+  # https://bbbot.dannydannydanny.me (VPS → ZeroTier → localhost:8080).
+  # The bot's start.py honors WEBAPP_URL to skip starting its own
+  # cloudflared Quick Tunnel when we've got a stable URL from the VPS.
   systemd.services.fitness-bot = let
     pythonEnv = pkgs.python3.withPackages (ps: with ps; [
       python-telegram-bot
@@ -131,7 +136,8 @@
     after = [ "network-online.target" ];
     wants = [ "network-online.target" ];
     wantedBy = [ "multi-user.target" ];
-    path = [ pythonEnv pkgs.cloudflared ];
+    path = [ pythonEnv ];
+    environment.WEBAPP_URL = "https://bbbot.dannydannydanny.me";
     serviceConfig = {
       WorkingDirectory = "/home/danny/tg_fitness_bot";
       ExecStart = "${pythonEnv}/bin/python start.py";