shipyard staging gets a stable URL: b3.dannydannydanny.me

Drop the cloudflared Quick Tunnel (URL changed on every restart →
unworkable for shipyard's apps.json). Move to the same pattern
every other tenant uses:

- vps-relay Caddy: new virtualHost b3.dannydannydanny.me →
  reverse_proxy to sunken-ship's ZT IPv6 :8081.
- sunken-ship: open port 8081 on the zt+ firewall interface
  (was 8080 + 8091, now 8080 + 8081 + 8091).
- fitness-bot-shipyard service: set WEBAPP_URL=https://b3...
  so start.py skips its own tunnel attempt; drop pkgs.cloudflared
  from path now that nothing in the unit needs it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nixos/hosts/vps-relay.nix

@@ -101,6 +101,11 @@
       "bbbot.dannydannydanny.me".extraConfig = ''
         reverse_proxy http://[fdd5:53a2:de33:d269:6499:93d5:53a2:de33]:8080
       '';
+      # B3Bot beta — bbbot's staging tenant under shipyard_poc_bot.
+      # Same backend host as bbbot prod, port 8081.
+      "b3.dannydannydanny.me".extraConfig = ''
+        reverse_proxy http://[fdd5:53a2:de33:d269:6499:93d5:53a2:de33]:8081
+      '';
       # Shelfish — phantom-ship's ZT IPv6.
       "shelfish.dannydannydanny.me".extraConfig = ''
         reverse_proxy http://[fdd5:53a2:de33:d269:6499:936c:48a:bbdc]:8081